The distinction is blurring. The Territorials were a separate army, but it's a bit academic whether they still have all the bits necessary to function as a stand-alone force. The Reserves have substantially been formed of ex-Regular servicemen who elect to be mobilised in time of emergency.
Now, there's a tendency to think of the Terriers as Reserves, even to the point that some units tag on a (R) sub-nominal instead of the traditional (V). Depending on your point of view, it's either ally or waltish.
Edited to add:
Think of the Terriers as having been a part-time army while reservists were individual replacements. The current vogue is to use the Terriers as individual replacements, hence the confusion.
Well I actually like it...my uncle, who joined the RA TA in 1938 said to me in 1979 "Ah Cuddles me boy, I see you have become one of the Terriers." It was as if in his mind there was a connection between the Terriers of 1908, 1914, 1939 and beyond.
I also like the fightiness implicit in the word Terrier - although no doubt given recent events in our urban centres "STAB" has all the fightiness the yoof require!