The unit for $\textit{Technical Atmosphere}$, symbol: $at$, is a non-SI unit of pressure equal to one kilogram-force per square centimeter.
The symbol $at$ clashes with that of the katal (symbol: $kat$), the SI unit of catalytic activity; a kilotechnical atmosphere would have the symbol $kat$, indistinguishable from the symbol for katal.
It also clashes with that of the non-SI unit, the $attotonne$, but that unit would be more likely be rendered as the equivalent SI unit.
Assay ton (abbreviation $AT$) is not a unit of measurement, but a standard quantity used in assaying ores of precious metals; it is $29 1D6 \,grams$ (short assay ton) or $32 2D3 \,grams$ (long assay ton),
the amount which bears the same ratio to a milligram as a short or long ton bears to a troy ounce.
In other words, the number of milligrams of a particular metal found in a sample of this size gives the number of troy ounces contained in a short or long ton of ore.