The terms are used interchangeably: both make a power supply behave like a battery so you can test chargers, BMS and converters without a real pack. The N35500 with NS81000 provides 7 standard model libraries plus custom measured curves.
What it models
State of charge, open-circuit voltage and internal resistance over the charge/discharge range — repeatable, safe and faster than cycling a physical cell.
Why bidirectional is required
A real battery both delivers and accepts current, so a credible emulator must source and sink — exactly what a bidirectional regenerative supply does.