23andme file format

What (if any) is the official file format name that 23andme uses. I’ve only seen it referred to as 23andme format. Are they complying with a more general format? If so, what is it? If not, why did they invent a new format?

According to this page: fileformats.archiveteam.org/wiki/23andMe

Raw genetic data is provided in the form of a tab delimited file (ZIPped up for distribution), containing the fields rsid, chromosome, position, genotype (e.g. rs3094315 1 742429 AG).

Comment lines begin with the # character.

The file name is of the form genome_Firstname_Lastname_20012345678901.txt, zipped as genome_Firstname_Lastname_20012345678901.zip.

What if the variant is a deletion or insertion, particularly of more than a nucleotide?
How could you differentiate an insertion of -/TT from a T/T snp?
Anybody has got a real sample file to look at?