Application scenarios of APB4

What are some application scenarios of APB4? Please brief me about this as I am working on APB4.
why PSTRB signal in APB4 have four bits?

PSTRB signal indicates which byte lanes to update during a write transfer. it shows that the bus contain valid data, when PSTRB[3:0]=1111. why we need bus instead of single bit PSTRB signal?
I am working on protocol checker VC of APB4 to which I have to test the assertions written. Does it mean I have to write test cases to verify my assertions?

I have mentioned some of the assertions in the protocol checker module. Now I have to "TEST MY ASSERTIONS" and I am not getting the exact meaning of this words. Can anyone explain me in brief what does...
apb protocol checker (assertions)

How can I get apb protocol assertions on arm official site? Thanks in advance, KMK
[APB] Assert timing of PSTRB and PPROT

Hi All, I have a question about assert timing of PSTRB and PPROT. I think APB4 spec (IHI0024C_amba_apb_protocol_spec.pdf) does not describe when PSTRB and PPROT should be asserted. I guess these signals...