100% sure there's another page in that guidelines document where they actually say what the unit is. If you've only been given this particular page, you should ask for the full branding document.
Later edit: looking at the image again, i guess what they're trying to say is to break the width of the page — whatever the page size is for any given material — into 15 columns for portrait work (PAGE WIDTH/15) and 15 rows for landscape work (PAGE HEIGHT/15). So yes, looks like a 1:15 ratio depending on page orientation.
As Lucian says, it’s impossible to know for sure without having access to the entire branding document. If there is more to the branding guidelines than what you’ve shown here and there is a specification somewhere in there, then that will obviously be the correct unit to go for.
Most likely answer
Barring an actual specification anywhere in the branding guidelines, the most likely interpretation to me is that they are using the term unit here for a reason: it’s meant to be an explicitly abstract design unit, not a specific, physical unit of measurement.
The grid indicates that in a square design 15 units wide (the top right illustration), the logo should be 3.5 × 3 units in size, and the header image should then be 15 × 7 units in size. Apart from the left and right sides of the header image which bleed to the edge, there should be a 1-unit margin around the entire design; the QR code and whatever that illegible line of grey text is should rest on the bottom margin (right and left side, respectively).
When making an actual design, the abstract unit would of course have to become a concrete unit of measurement, but as long as the ratios are the same, this shouldn’t be a problem. If your square design is 30 × 30 cm when printed, 1 unit is 2 cm, so the logo should be 7 × 6 cm, for example. If your design is 900 × 900 px in size, 1 unit will be 60 px and the logo will be 210 × 180 px.
This way, the branding guidelines only specify margins, sizes, etc., according to a grid based on the relative size of one element to all other elements, which makes it very flexible and usable in all kinds of situations.