ARM: What is the difference between APCS and AAPCS ABI?

I am currently porting a compiler written for ARM to a different target architecture. I found that for ARM there are two different types ABI: APCS_ABIand AAPCS_ABI.

I looked through it and referred to the ARM manual, but did not find any differences between them.

However, when implementing the compiler backend, two types are ABIshandled differently and have separate implementations.

By the way, APCSthis is an abbreviation for ARM Procedure Call Standard and AAPCSstandard procedure call for ARM architecture. (Don't they mean the same thing?)

So what's the difference between APCSand AAPCS? Why are two different types defined ABIs?