HP 15c - Manual (Page 175)

Section 12: Calculating with Matrices 175
Several matrix functions operate on the matrix specified in the X-register
only and store the result in the same matrix. For these operations the
contents of the stack (including the LAST X register) are not moved –
although the display changes to show the new dimensions if necessary.
For the > 7, > 8, and > 9 functions, the matrix
descriptor specified in the X-register is placed in the LAST X register and
the norm or (for > 9) the determinant is placed in the X-register. The
Y-, Z-, and T-registers aren't changed.
When you recall descriptors or matrix elements into the X-register (with the
stack enabled), other descriptors and numbers already in the stack move up
in the stack – and the contents of the T-register are lost. (The LAST X
register is not changed.) When you store descriptors or matrix elements, the
stack (and the LAST X register) isn't changed.
In contrast to the operation described above, the O| and l|
functions do not affect the LAST X register and operate as shown on the
next page.