Collections are virtual groups of photos you create within your catalogs. You can even create sub-collections within a larger collection. Each collection tells you how many photos are in it, and you can open them just like other contact sheets. Because this is a virtual group, adding or removing photos from collections does not move or delete the original photos.
Creating Collections
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.
Deleting Collections
You can delete or remove a collection - and any of its sub-collections, by right-clicking on the collection name and choosing Remove collection. Deleting a collection does not delete the actual photos in the collection or remove them from the catalog. It just deletes the virtual grouping.
Adding Photos to Collections
Select photos in a contact sheet. Right-click on the collection name and then select Add selected items here.
Alternatively, select a collection name and then right-click on photos you have selected in a contact sheet and either use the keyboard shortcut of Control-Command-B or choose Add to Collection:
Adding Photos from Another Parent Catalog
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 add the photos into the catalog.
Removing Photos from Collections
Just as you can add photos by right-clicking and using the contextual menu, you can remove photos the same way. Removing a photo from a collection does not remove it from any catalogs, even if you had to add it to that catalog in order to put it in the collection.
Finding Which Collections Contain a Photo
If you have a lot of photos and a lot of collections, it may get confusing to figure out which collections hold which images. Right-click on a thumbnail in a contact sheet and select Reveal in Collections to see which collections contain that image. 
Sub-Collections
Just as you can create 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 using the Create button. Alternatively, you can right-click on the collection and use the contextual menu.
Move a Collection or Sub-Collection
You can move collections within the same catalog. If you want to move a collection, for example under a new parent collection, you do this by:
- Select the collection you want to move
- Right-click on the new parent collection to make this a sub-collection or the catalog if you would like to make this a top-level collection.
- Select Move Collection...
