Read a mode one class at a time

First identify owner, owning group or others for the accessing user.
Then read that class only. Numeric modes use octal, base eight, with
digits 0 through 7. Within each digit, add the values of present rights.
Fresh paper activity: mode 620 gives group write without group read.
Explain why the owner still uses the owner class.

Assume ordinary mode checks only:
no ACL, root override, other privilege or ancestor access problem.
Class order: owner, group, others. Bits: read=4, write=2, execute=1.
Worked mode 600: owner rw; group none; others none.
Activity mode 640: owner rw; group r; others none.
Directory primer: r lists names; x traverses; w with x can modify entries.
File write and permission to delete its directory entry are distinct.
Activity fresh permission-note.txt: harmless practice note (newline).
Inspect before/after chmod 640; predict mode prefix -rw-r-----.
Question: a group member who is NOT the file owner accesses a regular
file with mode 640. Can that role write through the stated mode? yes/no