ARM v8 A64 instruction 32-bit variant usage

Dear sirs,

I'm studying ARM v8a architecture recently. I found that almost each A64 data processing instruction has a 32-bit variant using sf to encode it. My question is as follows.

1. who is responsible for using this variant, hardware designer or software designer or only compiler designer?

2. when do we to use this variant, in processor design stage or software development stage?

3. Do all ARM 64bit processors support this variant?

4. How do we to choose this variant?

THanks.