The command USER_SELECT prompts the user to make a selection. When doing so a filter is applied, that let's you choose only certain objects. The selection is saved in a variable.
USER_SELECT Filter Variable
USER_SELECT_OPTIONAL Filter Variable
Filter = AXIS | FACE | AXIS_FACE | AXIS_POINT | POINT | CURVE | CSYS | CURVE_END
Example:
USER_SELECT POINT REF_POINT
The user gets prompted to choose a point. The point will be saved in the variable REF_POINT.
USER_SELECT_OPTIONAL defines a optional reference. If it's not needed the process can be cancelled with middle mouse button or cancel
The commapd USER_PARAM_INPUT guives the user the possibility to enter a DOUBLE value and store it in a variable for later use.
USER_PARAM_INPUT Variable