"Display..." tools in commands bar
The second drawer of commands bar is labeled "Display" and pretty sure relates to displaying aspects of objects, vertices and such. From first lesson you've learned how to show and hide particular objects in objects bar, but this is not the only way to make your workspace more clear for modelling certain details. In certain cases, you might have one complete detailed mesh object and will need to hide a part of it (e.g, hide the whole rear part of the vehicle to create some details on front grill). Display tools serve this purpose - they allow to hide/show elements of an objects as whell as the objects themselves.
Another important display aspect is a Disabled state. This is a state of an object/vertices/polygons when they are visible, but they are not accessable for modifications (you can't even highlight such an objects). These objects are still visible, but shaded in gray color. Benefit of disabling comparing to hidding is that you actually can see the mesh. So, the display tools follow.
Disable tool used to disable objects and sub-elements. It can be applied in selected mode too. To disable any element, highlight it and click on it. When applying in selected mode, simply click somewhere in viewport.
Enable obviousely enables objects and sub-elements. Since you can't highlight disabled elements, this tool enables all disabled objects (if you are on objects level) or all vertices/polygons (if you are on sub-level) immediately when you press this button in commands bar.
Hide tool used to hide objects and sub-elements. Benefit of this tool is that you can completely hide desired set of elements (e.g. a group of selected vertices or polygons). By default 'H' hotkey is assigned to this tool.
Show tool restores objects or their elements from hidden to visible state. Since you can't see elements that you are going to make visible, all you need is to click this button in commands bar.
Hidden sub-elements (vertices or polygons) are hidden only when you are on according level. This means, that vertices keep hidden only when vertices level is selected. If objects or any other level is selected, vertices will be visible. Note, if you switch object to vertices level and part of it's vertices or even all vertices disappear, this means that these vertices are hidden. Click on Show button to turn them into default state. All above applies to polygons too.
This tool is used to change visual state of several objects at a time. The best choice for this tool when you have a really huge amount of objects in scene, or if you want to show/enable only few of previousely hidden/disabled objects. Title of this tool is in square brackets - this means that clicking on the button will pop-up a dialog box. An example of this dialog box is on the right. Two options areas at top allows you to select which nodes will be listed. You can list hidden objects if you are going to show them up, or you can list enabled objects if you wish to disable some of them. On the image enabled and hidden objects are listed. Once options are set, list content is filled with objects that fit criteria. Also, I've typed door in "Quick select" field and objects with particular match in name have been selected in the list. From this point any of four buttons in the bottom can be pressed to change visual state of "selected" objects. Changes will take effect immediately. When done, OK button should be pressed (Cancel button will obviousely discard all changes).
Once dialog has pop up, you can perform several operations: you can hide one set of objects, unhide another set of objects and disable third group of objects. And only then press OK button - you don't need to pop this dialog three times.
The tools under this branch affect object's local axes. In most of cases local axes are aligned to world axes (axes direction oriented the same way) and optionally can be moved to certain point. Most games use only position of local axes (this can be a mounting point for an objects or effects), but sometimes local axes orientation (direction of axes) is important too. In some games certain parts of the model are rotating around one of the local axis: swing-up doors, helicopter rotors, engine's cooling fan - these parts can rotate around certain axis, so local axes should be oriented properly. Local axes can be moved and rotated with Modify\Move and Modify\Rotate tools respectively, but this branch of tools brings the fastest way to perform very common operations on axes: reset to world, reset to parent, copy (from one node to another) and center axes to object.
An options box in Local Axes... button has one switch option: offset or orientation and one on/off option keep object. The switch selects whether you would like to alter location (offset) or direction (orientation) of axes (you can not alter them at a time). "Keep object" option, when turned on, allows to keep object geometry unchanged. If not set, object geometry will move and rotate with it's axes too.
This tool, obviousely, centers local axes to object. If you have moved axes too far away, or altered an object and want axes to be in the middle of an object - this tool is your choice. This tool is also helpfull when you need axes to be exactly centered on object. Vehicle wheel object is an example - in most games it's axes have to be exactly centered, or otherwise it will spin improperly.
These tools reset axes (orientation OR offset) to parent (in hierarchy) or to world axes respectively. If you need axes to exactly match parent or world, these tools have to be applied twise - one time with Orientation selected in "Local Axes..." options and one time with Offset selected.
This tool allows to copy local axes orientation or offset from one object to another. When this tool is selected, click on the source node and move cursor to destination node. A dashed line with an arrow will be drawn. Once you highlight and click on destination node, local axes will be copied. If you need to copy both orientation and offset, you have to repeat this operation twice with different switch options set in "Local Axes..." box.