How to upgrade a system on SAP S/4HANA 1610 from a Feature/Support stack to a higher one

Hello. Thank you for the RIG document. But it is still not very clear as to why certain options were chosen the way they were chosen. Please let me ask a few questions here that may help to clarify

a) When we update the S4 1610 backend from say FPS01 to FPS02 - DO we HAVE to (mandatorily) update also the S4 Fiori frontend ?

b) I assume the answer two a) is yes. If yes, which parts of the Fiori Front End would need an update?  only the SAP FIORI for S/4 HANA 1610 ? or also the Fiori Front End server (SPS) ?

c) how about viceversa. Would there be cases were a certain S/4 HANA Fiori UI would need an SP without having to update anything on the backend. I assume yes. Is this a common and recommended approach or is it advised to only do a complete SPS update on the fiori front end server?

d) does the concept of FPS apply to Fiori Front end server also ? or does it only apply for S/4 backend and on the Front End its only SPS and not FPS?

e) The maintenance planner options for the S/4 backend in the document was fairly clear. We select FPS01 to FPS02 . Nothing fancy there. But when it comes to maintenance planner options for S/4 Fiori front end - this is a bit confusing  --

1. First you select 'Update SAP Netweaver' and select SPS01. Why ? Why is it not setup in same way like S/4 backend where you select S/4 1610 and choose FPS02 and it automaticlaly updates the underlying SAP Netweaver also?
2. Continuing with the same question above, why do we choose only SPS01 here. is there any version equivalancy ? Like we chose FPS01 for S/4 backend so on the frontend it can only go till SPS01 ? Is there any note that explains this version dependency?
3. Why do we do Install and maintain Addon - seperately. And why do we select both Fiori for S/4 HANA 1610 and Fiori Front End server here? Is it not sufficient to just choose S/4 HANA 1610 ? do we really need to update the FES also?  it seems to much of manual selection options with the possibility of human error
4. and finally in steps 9) and 10) of section 4 - its not clear why we choose again only SPS01 within FP1 ? Is there any documentation explaining why we should not choose say SPS02 ? Here in this case perhaps SPs01 was the latest released but if there were more, would maintenance planner allow us to choose whichever SP we want? Is there no note explaining what is the right option and version dependency?

thx