While the standard BARM references is the final say, simplistically with metric units...
If the unit is named after a person it's a capital, eg J for Joules and N for Newtons.
Otherwise it's lower case, eg m for meter/metre and s for second.
As regards multipliers, the case also has significance k for kilo M for mega, m for mili etc.
So even on drawings meters are m, kilometers are km and millimeters are mm.
KENAT, probably the least qualified checker you'll ever meet...