Taken from the manual:

“Plus-P” (+P) ammunition generates pressures in excess of the
pressures associated with standard ammunition. Such pressures
may affect the wear characteristics or exceed the margin of safety
built into some revolvers and could therefore be DANGEROUS.
This ammunition should not be used in Smith & Wesson
medium (K frame) revolvers manufactured prior to 1958. Such
pre-1958 medium (K-frame) revolvers can be identified by the
absence of a model number stamped inside the yoke cut of the
frame (i.e., the area of the frame exposed when the cylinder is
in the open position).
“Plus-P-Plus (+P+) ammunition must not be used in Smith &
Wesson firearms. This marking on the ammunition designates
that it exceeds established industry standards, but the designation
does not represent defined pressure limits and therefore such
ammunition may vary significantly as to the pressures generated.
Some brands of ammunition may cause difficulty in extracting
spent cartridge cases from the cylinder or chamber. If this
situation occurs, thoroughly clean the cylinder charge holes or
chamber with solvent. If this condition persists, we recommend
changing to another brand of ammunition.