A lot of the SoftDevice documentation I have been reading indicates that "Appendix A: SoftDevice architecture” in the nRF51 Series Reference Manual is prerequisite reading but I couldn't find this Appendix in the reference manual that I had downloaded from the nordicsemi website.
It appears that it is just missing from v3.0 of this document. I tried downloading the previous version (v2.1) and I did find it there. I found the older versions available here
Was this appendix deleted by mistake when the "Software Interrupts (SWI)" chapter was added to the latest reference manual?