I don't know why searches did not find this for me, in the various places and ways I looked.
However What does that mean of PgIn[ST3P], RdMeta[ST3UP] 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