Each output line is constructed according to the FORMAT in the -o option. Each element in FIELD-LIST is either the single
character 0 or has the form M.N where the file number, M, is 1 or 2 and N is a positive field number.
A field specification of 0 denotes the join field. In most
cases, the functionality of the 0 field spec may be reproduced
using the explicit M.N that corresponds to the join field.
However, when printing unpairable lines (using either of the -a
or -v options), there is no way to specify the join field using
M.N in FIELD-LIST if there are unpairable lines in both files. To
give join that functionality, POSIX invented the 0 field
specification notation.
The elements in FIELD-LIST are separated by commas or blanks.
Blank separators typically need to be quoted for the shell. For
example, the commands join -o 1.2,2.2 and join -o 1.2 2.2
are equivalent.