Collections allow you to create virtual groups of photos within your catalogs. You can also create sub-collections within a larger collection. Each collection tells you how many photos are in it, and you can pull them up just like other contact sheets.

When you 

Create a collection by using the Create button or right-clicking in the collections section.

You can only create collections within catalogs that have Add/Modify enabled. 

Once you have created a collection, you can add photos to it, even if the photos you want to add are stored on media that is not online. 

Select photos from a contact sheet. Right-click on the collection name and then select Add selected items here. 

In order to add items into a collection, they must first be in that collection's catalog. If you try to add photos to a collection in a different catalog, Photo Mechanic Plus will offer to copy the photos into the catalog. 

Sub-Collections

Just as you can add collections, you can create sub-collections within a larger collection. This acts as a virtual folder within your collection, but as with collections, it does not physically move your cataloged photos. 

Create sub-collections by selecting the desired parent collection and then use the Create button or right-clicking on in the collections section.