A selection-expression represents an arbitrary collection of atoms over which some operation is typically applied. It is one of the two types of open-ended arguments many PyMOL commands accept. The other is a name pattern.
For example, one might wish to uniformly color all carbons in all molecular objects uniformly using a single command:
color white, (symbol C)
In the above example “(symbol C)” is a selection expression, constructed using PyMOL's atom selection language.
The parentheses are optional, but they help to distinguish selection expressions from name patterns. When PyMOL accepts either a selection expression or a name pattern as an argument to a given command, the input will be treated as a selection expression if the argument contains parentheses or a recognized selection keyword.
Since non-molecular objects do not contain atoms, they cannot be specified using selection expressions. Thus, selection expressions only apply to molecular objects.