Complex sentence: If an RFC becomes an Internet Standard (STD), it is assigned an STD number but retains its RFC number; however, when an Internet Standard is updated, its STD number stays the same, now referring to a new RFC or set of RFCs.
Split into simpler sentences: If an RFC becomes an Internet Standard (STD), it is assigned an STD number but retains its RFC number. When an Internet Standard is updated, its STD number stays the same, now referring to a new RFC or set of RFCs.