TM500 high-power compartments
A metal cover to the left of the backplane connector pushes a switch in the rear of a high-power plugin in that compartment.
In the other, low-power compartments, a hole in the cover prevents this. Smaller mainframes like the TM503 may have no such covers at all.
In the high-power slot, the 25 V transformer windings can supply at least 1 A each instead of 0.5 A. The +11.5 VDC/17 VAC supply is separate from the rest of the slots and can float, it is rated for ~4 A instead of 1.3 A. See the TM500 Series plug-in interface for more details.
The PS503A will deliver ±20 V at 1 A in a high-power slot, instead of only 0.4 A. The PS505 must be operated in a high-power slot to deliver its rated 4 A output, otherwise it is limited to 1 A (and the output is not floating).