It’s named AltSoftSerial because it is a software-based serial library that is an alternative to the NewSoftSerial package.

[Jacob Geigle] had a problem. A GPS unit and a Bluetooth-to-serial were tying up all the hardware UARTs on an AVR Arduino project. “Software serial”, I hear you say. But what if I told you ...

It's of course backwards compatible with existing Arduino add-ons, but sets itself apart with a new USB controller with lower latency and driver-free operation on Mac and Linux.