Rest Patterns Book

Help with guitar strumming rhythms?
I received a guitar for Christmas and I am learning to play it. I also got a book called “The Complete Guitar Player Songbook: Omnibus Edition Two.” So far I can understand most of the music in the book, except for the strumming patterns. It uses a lot of up and down arrows(which I can get, strum up strum down). But in later songs it starts using T, i, m, and r and I can’t figure out what this mean (for example:
arpeggio T i m r T i m r
1 & 2 & 3 & 4 &
I understand the term arpeggio and even the 1 & 2 stuff, but what does the rest mean. It’s really frustrating, the book doesn’t say and I can’t find this information anywhere.
I have the same book, and can find many example of the 1 & 2 & 3… but not the T i m r, Plenty of T which refers to thumb picking the base note, while finger picking the others. The i m r may refer to the other fingers used to pick, ie index, middle, and ring.
what page, and songs do you see this on?
Classical Guitar Lesson #1: positioning the guitar