use commas (FALSE, 0, or omitted): Include position separators in the result.
no commas (TRUE or 1): Don’t include position separators in the result.
Examples
=FIXED(6789.123, 2) returns "6,789.12."
=FIXED(6789.123, 1, 1) returns "6789.1."
=FIXED(6789.123, -2) returns "6,800."
=FIXED(12.4, 0) returns "12."
=FIXED(12.5, 0) returns "13."
=FIXED(4, -1) returns "0."
=FIXED(5, -1) returns "10."