What is AT-300 type hydrotest for pressure vessel?

I believe you are referring to the "old" code paragraph:

AT-300 HYDROSTATIC TESTS BASED ON
VESSEL DESIGN PRESSURE
Except as otherwise permitted in AT-330 and AT-
400, completed vessels designed for internal pressure
shall be subjected to a hydrostatic test pressure which,
at every point in the vessel, is not less than 1.25 times
the design pressure (see AD-121.1) to be marked on
the vessel, multiplied by the lowest ratio (for the
materials of which the vessel is constructed) of the
stress intensity value Sm for the test temperature of the
vessel to the stress intensity value Sm for the design
temperature (see Table AD-150.1).