understanding the relationship between Zip Codes and ZTCA's

I am trying to get my head around what exactly the ZTCA's represent and how
I can relate some demographic data I have in regards to zip code to them.

I have a table of data which is the number of patients served by my clinics
from each zip code.
I have a table of ZTCA's which I understand are an attempt to map zip codes
to census areas. Since Zip Codes aren't necessarily an AREA this means that
some zip codes arent' represented in my ZTCA table. Is there a best
practice for representing these non-existant zip codes on my map?

Then I have my 999XX and my 999HH ZTCA's. What the heck are these? I read
the information on the census bureau page and I still dont' get it. Call me
dense. Call me whatever. But explain this to me so that I understand whats
going on.