I honestly don't think these qualify as 'Common'. And it's already in the comments:
From plugin-defaults.ini:
Code:
## Literal strings can be set into custom columns using double quotes.
## Each metadata=>column mapping must be on a separate line and each
## needs to have one space at the start of each line. There is also a
## special case for Integer and Float type custom columns: The static
## value "None" will clear the column.