A collector is a yellow button on a panel that enables you to select an entity type either graphically or from the Select an "Entity" Dialog.

A panel can contain any number of collectors, but only one can be active at a time. Entities can only be selected from an active collector. A collector is active when it is enclosed in a blue rectangle, as shown in the image below.
Figure 1. Body and point collectors

The label on the collector indicates the current entity type. Most collectors also display the name of the current selected entity in the field next to it. If no entity is selected, the field will display the label "Unresolved".
Figure 2. Collector label

Some collectors have a collector switch that allows you to change the entity type from a combo box. When the combo box is expanded, a list of available entity types is displayed. Only one type of entity can be selected at a time.
Figure 3. A collector with a combo box