Units are selected for beneficial or nominally benign actions. This is the same thing as targeting, and the terms are used interchangeably, although the implied intent is different.
Left clicking a PC, NPC, or mob will select it. Right clicking is used for more direct interaction; it selects and will open a dialog with a friendly NPC if the NPC has a dialog for you, but it will target and auto-attack a hostile NPC or a neutral or hostile mob if it is within range of your melee weapon(s), or if it is within range of your ranged weapon if you are playing a hunter and have autoshot turned on. The tab key also targets and selects, and macros can step through targeting/selection choices.
Neutral NPCs vary in how they respond. Neutral vendors will generally open a dialog. Many are not selectable.
Selecting is used for:
- Buffing an individual
- Healing an individual
- Removing a Debuff from an individual
- Chatting with an individual player character
- Opening a trade with a friendly player character is a right-click action that selects that PC
- Examining a friendly player character is a right-click action that selects that PC
- Interacting with objects
- Testing whether certain objects, like anvils and forges, will function for crafting. Some are merely decorative and those cannot be selected.
- Hunters select a mob to tame it, but taming acts like an attack until the mob is tamed. The mob being tamed turns hostile if it is neutral.
Friendly player characters can be selected for any of these.
Friendly and neutral NPCs can often be selected, and then buffed or healed.
Hostile PCs can be selected for chatting. They can't understand you directly, but they can hear your babbling and see your emotes. It is useful to remember that selecting a hostile PC to send an emote is the same as targeting them.
Selecting a mount that is being ridden will select the player character, not the mount. The mount is in effect a rather large extension of the PC under the PC.
A selectable object (or unit) in front of another selectable object (or unit) will block the ability to select the further object. If it fully covers the other object, that object cannot be selected by clicking on it. For example, many players have recently discovered that parking a large mount on top of a mailbox blocks access to that mailbox. Units can still be selected by using the tab key or macros.
Units which cannot be selected cannot be interacted with directly.