Maximum or Optimum Loading of a Transformer

...........it's when the output impedance of the transformer matches the input impedance of the load.

Depends on the type of transformer; Distribution transformers 60-80%.

Data or power transmission, which specs ??

(there is no general answer on that question !!)
Regards Uwe

Depends on the regulation of the output and the type of lamination and core used. Normally transformers are designed for 25% over load factor so that at the required ouitput voltage & current the regulation is within specified limits.

every heard of the term maximum power transfer

when the impedance of the source equals the impedance of the load

z1=x1+y1 ,z2 = x2 + y2;

transformers:

http://www.tpub.com/neets/book2/5i.htm

If you are from India refer CBIP Technical Report 9 where mathematical formulation is used to describe temperature rise and loading of transformer. Also, IEC 60384(so far I remember) gives good discusion.

That's IEC 60354 (not 60384)