it should be simply temperature rating of the windings (just like with motors)
to prevent insulation breakdown. it simply means that intended operating temperature
is no more than 105degC inside the transformer (not the surface).
there are many standards that cover different electrical products and
i'm not sure about transfrmers since i don't really make them.
it also depends where you live. for example in USA there are tons of UL standards,
Europe is using IEC etc. they are all fairly close but everyone has something different.
standards we follow are not free. in fact every little brochure can easily cost $100 or more
and the bigger once are more expencive. you can call and ask if UL1740 covers it.