According to https://www.zabbix.com/documentation/current/en/manual/appendix/suffixes, '600M' should be the same result as '600000000'.

But actually for any item (with user custom suffixes or without them) cause 1024-base calculation to display a trigger line on grpah.

See:

Even if you apply B as the item unit, which cause axis scale re-presented accordingly, the trigger line is still displayed higher than a value line, which is even more unexpected, as it indicates that 1024-base is applied on top of already applied one:

p.s. this description had been rewritten, so ignore initially provided graph with green/blue filled regions.