mTSV is a simple, plain-text file format.
It is very similar to the TSV file format — with the only difference being the addition of invisible lines.
Both file formats represent a table of data.

To make this specification simple: each field in the table is separated by a TAB character ('\t' or 0x09).
Each record in the table is separated by a LINE FEED character ('\n' or 0x0A).
If a line begins with an OCTOTHORPE character ('#' or 0x23) then it can be ignored (that is to say, treat it as a comment).

An example file is shown below (note that the TAB may appear as spaces, depending on your browser settings):

# this is an example mTSV file.
# each of these lines CAN be ignored by a parsing library.
record 1, field 1   record 1, field 2   record 1, field 3   record 1, field 4
# “invisible lines” can on any line in the file.
record 2, field 1   record 2, field 2   record 2, field 3   record 2, field 4