The radar and Electronic Warfare communities generally accept some commonly used notation for the various parameters used in radar and EW calculations. Textbooks and reference handbooks will usually use this common notation in formulae and equations.
Subscripts are a different matter.
Subscripts are often whatever seems to make sense in the context of the particular formula or equation. In the following list, generally accepted notation is given in the left hand column with no subscripts. One way space loss, target to radar receiver, bistatic Other notation such as tm may be used to clarify specific losses, in this case the space loss between a target and missile seeker, which could also be identified as 1r .