HP 15c - Manual (Page 105)

Section 9: Subroutines 105
Further Information
The Subroutine Return
The pending return condition means that the n instruction occurring
subsequent to a G instruction causes a return to the line following the
G rather than a return to line 000. This is what makes a subroutine
useful and reuseable in different parts of a program: it will always return
execution to where it branched from, even as that point changes. The only
difference between using a G branch and a t branch is the transfer
of execution after a n.
Nested Subroutines
If you attempt to call a subroutine that is nested more than seven levels
deep, the calculator will halt and display
Error 5 when it encounters the
G instruction at the eighth level.
Note that there is no limitation (other than memory size) on the number of
nonnested subroutines or sets of nested subroutines that you may use.