They are international standard unit-of-measure codes (UN/ECE Rec 20): KGM kilogram, TNE tonne (metric), MTK square metre, MTQ cubic metre, H87 piece.
Units are picked from a list rather than typed in. The reason is simple: "m2", "m²", "sqm" and "square metre" all mean the same thing, and once they are spelled differently the offers can no longer be compared. A fixed code list removes that mess — and these are the same codes that appear on customs and e-invoicing documents, so buyer and supplier are measuring the same thing.
You will see the codes, but you never have to memorise them. On the Products step of the demand wizard, open the Unit field with Select a unit, then type into the search box (Search units (e.g. square metre, kg, piece)…). You can search by name, by symbol or by code: typing "square metre" finds MTK, typing "kg" finds KGM. Each row shows the symbol (m², kg, t), the unit name and the code together.
The list covers the area, weight, length, count, volume and time units used in stone and building-material trade — for example MTR metre, LM linear metre, PF pallet, CTN container, SH sheet, DAY day.
Also asked as: What does KGM mean? · Which unit is MTK? · What are these codes in the unit list? · Why does it say MTK instead of square metre?