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 (a process may mark its I/O as passive; passive I/O may be throttled but doesn't cause throttling of other I/Os.)
PS my other questions still open.
For speedier output to a log file use
mawk -W interactive