Graphics subsystems in R

Question

The R Internals documentation on the schedule (as of 2016/03/06) mentions that

Despite the fact that since 2001 there have been 24 subsystems, at present there are still only two bases: "base" and "grid"

What does it mean? I understand the difference between subsystems gridand graphics, but why is there a limit on the number of alternative subsystems?

Thank you for your help.