In zOS 1.3, in the SPQE block, there is this pointer to the SPQA block.
There is also a pointer to the SPQX block.

I know what the SPQA block is is meant for.

I don't know what the SPQX block is meant for.

Can someone explain ?

Thanks

It's all part of the reorg of control blocks to keep
track of the various categories of virtual storage
now present since the advent of 64-bit addresability.
These changes were introduced with OS/390 2.10.
Have a >good< look in SYS1.MODGEN(IHASPQA).