I don't know why searches did not find this for me, in the way I looked. however
https://apple.stackexchange.com/questions/299175/what-does-that-mean-of-pginst3p-rdmetast3up-when-use-fs-usage-in-osx

gives some useful data and even the source code.

A or S = async / sync

N = nocache

T is optionally followed by a number: this is something to do with P_DISKIO_TIER_MASK and P_DISKIO_TIER_SHIFT... 
and then this is optionally followed by U, indicating P_DISKIO_TIER_UPGRADE

P = passive

PS my other questions still open.

For speedier output to a log file use

  `mawk -W interactive`