Rockwell Automation 1772-L8_LW_LWP_LX_LXP,D17726.5.8 User Manual - Manual (Page 265)

Jump Instructions and
Subroutine Prorgramming
Chapter 17
17-6
The boundary marker subroutine area appears. A Subroutine Area
instruction can only be programmed after the last instruction in the main
program. The Subroutine Area instruction cannot be inserted between
rungs in the main program. It requires 1 word of memory, it can be
programmed only once, and it cannot be removed except by clearing the
memory. You cannot nest subroutine programs by inserting a Jump To
Subroutine instruction within the subroutine area, but it is possible to jump
from one subroutine to another using a Jump instruction.