While reading an e-book about transformer protections and relaying, I found this good information for Power Engineering newbies like me. And I quote,

“This chapter describes the protection practices for transformers of the following types whose three-phase bank rating is 501 kva and higher:
Power transformers
Power autotransformers
Regulating transformers
Step voltage regulators
Grounding transformers
Electric arc-furnace transformers
Power-rectifier transformers”

This simply implies that according to power systems protection’ s point of view, any transformer whose three-phase bank rating is 501 kva and higher is considered as power transformers while any rating below 501 kva are called distribution transformers