This is the range, you can find out more about it in the documentation.
All – Retrieve all objects at once.
First – Retrieve only the first object. The result of the retrieve action will be a single object instead of a list. Note that when you know you have only one object or you only want to use the first object in a list, use First to get a result of the object type (as opposed to a list).
Custom – Retrieve a given number of objects (Amount) starting at a given index (Offset). The amount and offset are expressions that should result in a number. Note that the first object has an offset of 0. An amount of 0 means that all objects are retrieved.
I hope this helps.