Updated: Mar 4
Linear regulators have a fix or adjustable voltage at their output, which always is less than the input voltage. This is usually marked as Vdrop (Voltage drop) in the datasheets and means, which voltage is "cut off" from the input voltage. The Vdrop usually depends on the current need - the higher the current, the higher the Vdrop.
The voltage difference between input and output voltage multiplied with the current (that the powered device consumes) results in VA of generated heat, which has to be dissipated by the heatsink into the environment.
Let's assume, that your device consumes 3 ampere, then it's clear, that the bigger the voltage difference between input and output is, the more heat is generated.
For DC-powered HPULN-PS's we recommend a guaranteed voltage headroom of 0,6V under full load delivered by your pre-reg.