The collection dialog is located at [Menu bar --> Tools --> Collections]. Alternatively, it can also be accessed through its toolbar button:
[Collection toolbar button]
Add new collection: adds a new, empty collection. Always try to give a unique name to your collections in order that name clashes won't occur when composing your scene with other models. Collections can be renamed in the collection list.
Visualize selected collection: allows to visualize the selected collection by selecting all related objects in the scene.
Operation on selected collection: this section of the dialog allows you to add or subtract various objects to/from the selected collection. When you add a new collection, it is initially empty. You can then add or subtract individual objects (Selected objects), all objects contained in a hierarchy tree (Tree of selected object), all objects contained in a chain (Chain of selected object - i.e. all parents and ancestors of the selected object), or all objects in the scene (All objects).
Composing elements and attributes: list of elements that compose the selected collection. Individual elements can be deleted with the delete-key.
Collection overrides collidable, measurable, renderable, cuttable & detectable properties: allows overriding the main properties (collidable, measurable, renderable, cuttable & detectable) of the objects in the collection. If for instance you don't want some objects to be detectable, except for rare or special cases, then simply disable the detectable property for those objects (refer to the object common properties dialog), then group those objects in a collection with this item enabled. Now those objects can be detected if and only if they are referred to as the collection you just defined (in other words, they can be detected only if you try detecting their collection instead).
Object common properties dialog