An assembly or sub-assembly is a unit that is comprised by multiple components. In
a circuit this sub-assembly must be surrounded with a pink dash-dot line on ISO_LIN_ASS layer; you draw it with the Envelope command.
Example, you could have a manifold block (sub-assembly 1000) and a power unit (sub-assembly 2000) in the same circuit diagram. You want to number the components in sub-assembly 1000 to be 1, 2, 3 and so on. Also you want to name the components in sub-assembly 2000 to be 1, 2, 3, and so on. In this case you use the Assembly ID in the command Reassign Item ID for both sub-assemblies.
When you then create a parts list of the complete system, the components in the manifold are named 1001, 1002, 1003 etc. and the components in the power unit 2001, 2002, 2003 and so on to show that these components belong to the respective sub-assembly.
Note, you have to create a separate text in the drawing for each sub-assembly, like Assembly 1000 for the manifold block. It is not created automatically.