Convention to label components in a circuit
There is no standard convention, or even a widely-used ad hoc approach.

These labels are called reference designators. You must make sure that the first letter is correct for the type of component being labeled...that much is standardized.

Do not reuse reference designations...if you delete R1 from the schematic you should never have another R1. You should also maintain a list, somewhere on the drawing, of reference designators that are not used because their corresponding components have been removed.