WRITING IS EASY
Symbols like mL or mg or mm or kg or cm together
identify Prefix and everyday unit of measure.

Symbols
identify prefix and base or derived unit
OR everyday unit of measure together.
Simply place the prefix in front of the base or derived unit
like mm (millimeter), cm (centimeter), km (kilometer)
or mg (milligram), kg (kilogram) and mL (milliliter).

The size or quantity is indicated by the first letter or prefix symbol and the second letter or symbol identifies
the base or derived unit.
 Please Note: Where symbols are used convention has it that they are always lower case letters except where the Base Unit is named after a person like Celsius (°C), Pascal (P) or Newton (N).
 Then the symbol is noted by a CAPITAL LETTER.
"So what's with a capital letter L for liter?
    Is that a capital "L" for the unit of measure named
after a person?"
No....
    You see, when decimal dimensions were described long before the typewriter you would have to write the script.
    But as technology came along it became apparent that typewriters were not equipped with the script so attempts were made to use the lower case "l" (el).
   Then it became further confusing to write "1l"
(lower case "el") as the "1" and "l" (el) looked
too similar as did the capital "I" (eye).
   So the capital "L" was chosen as the symbol for
the derived base unit "liter".