To make things more confusing people will talk about steam in terms of mass and pressure. As Dag Otto has mentioned a pound of dry steam has a specific heat value. There are other ways that people will think of steam pounds though:
When talking about steam people will talk about the pressure of the system, expressed in pounds per square inch gauge (PSIG). Or more commonly referred to as ‘pounds’ of steam.
Steam systems come several types. The most common systems that people will see are the home heating systems. Formally these systems are known in civilian practice as low pressure systems. They’ll operate at about 1-5 PSIG, and for many jurisdictions the limits for a low pressure system is that it can have an operating pressure no greater than 15 PSIG. This is likely what your house operates at, if it’s above atmospheric pressure at all.
Above 15 PSIG, and to about 125 or 150 PSIG, you get the ‘mid’ pressure systems. Not all jurisdictions recognize this, though. This is the realm of larger buildings, that need more driving power to get through all the floors they have. It’s also the beginning of the more dangerous steam accidents. Since we’re talking about saturated steam, here, the pressure also is a description of the temperature of the steam. It’s a lock-step relationship. The 15 PSIG system is going to be about 240 F, IIRC. The 150 PSIG system is going to be about 350F - with the potential for damage to people and property rising dramatically with the rise in temperature.
Above 150 PSIG is the realm of high pressure steam. The temperatures get pretty high, up to about 650 F, IIRC. (Above that you start dealing with plasma, not steam anymore.) The pressures get similarly high. A steam turbine plant, using saturated steam, will operate at about 600 pounds, for example. But they’re not the only ones who use high pressure steam. Most dry cleaners will have a boiler operating at around 150 PSIG.