Why AWLEN signal is optional for master and required for slave in AXI4 protocol ?

Why AWLEN signal is optional for master in AXI4 protocol ? If it is optional for master then how slave will get that signal because it is required for slave ?
Why does AHB or APB support only 16 slave devices?

I would like to know why only 16 slave device select lines are present in the AHB and APB bus architectures.
Why does AHB or APB support only 16 slave devices?

I would like to know why only 16 slave device select lines are present in the AHB and APB bus architectures.
HTRANS for HBURST == SINGLE

Is HTRANS always NONSEQ for SINGLE bursts? I imagine this is ok: A0 A1 A2 NONSEQ NONSEQ NONSEQ SINGLE SINGLE SINGLE I imagine this is not ok (?): A0 A1 A2 NONSEQ SEQ SEQ SINGLE SINGLE SINGLE
HTRANS for HBURST == SINGLE

Is HTRANS always NONSEQ for SINGLE bursts? I imagine this is ok: A0 A1 A2 NONSEQ NONSEQ NONSEQ SINGLE SINGLE SINGLE I imagine this is not ok (?): A0 A1 A2 NONSEQ SEQ SEQ SINGLE SINGLE SINGLE