I've helped write an XML Schema in the past which is roughly similiar to this.
My advice would be to be absolutely explicit in documenting exactly how the file is constructed, so no ambiguity exists.
So things which you need to consider are:
- Spacing
- End of line markers
- End of file markers
- Allowed characters in each area of the file.
- Allowed value ranges in each area.
- Any encoding which is required, and what method/algorithm to use.
Ideally you can write a validator program to give to the third party, to validate files. Thus minimizing the chance of something going wrong.