dedicated return line and recirculation pump
Some recirculation systems are based on gravity/thermal expansion effects that create a small pressure differential to keep hot water moving and recirculating through the pipes, from the hot water output back to the recirculation input.
This is a feature for comfort, and it comes at the expense of efficiency: hot water continually leaves the heater (albeit only a very low flow) and returns a bit less hot after having been cooled by the piping, all this while never having been used for washing etc...
This same gravity/thermal expansion effect that is exploited for recirculation can also cause an undesired flow of hot water out of the heater. Hot water leaves the heater and returns to it a bit less hot, all in the same outbound pipe. It's convection within a pipe, much like convection in a tall and narrow upright cylinder.
This undesired convection can be stopped by installing a P-trap or a nipple. Both create counter pressure to stop the flow: the P-trap by the gravitation and counter convection by the P shape of a section of pipe, and the nipple (a short piece of pipe with two male threaded ends) by the elasticity of a membrane or a ball inside it. It will take more pressure than that provided by convection to cause water to flow through them.
Two traps are shown here, one is based on a heavier than water ball, the other on a membrane.

When a faucet is opened, the much larger pressure from the water mains will easily push through the minor counter pressure, allowing water to flow as expected, but only when a fixture is open.
The counter pressure to prevent convection losses can form a problem for recirculation pumps that are designed for low flow at low differential pressures. Thus, a recirc pump that intends to pump water at a very low rate through a heater with nipples in the lines will need to be slightly more powerful than one rated for lines without such counter measures.
So if a pump is not rated for heat traps, it means it cannot overcome the heat trap's counter pressure. If the heater or its installation enable recirculation based on the same gravity principle, then the pump most likely can push through it, as it will aid the gravity induced flow, not counter it.