Understanding transformer datasheets
This transformer is made for LT3420, which is a flyback controller so this gives you some input to how you might specify the transformer, but I agree it should state verbatim in the datasheet.



Designing with flyback means your primary inductance (magnitizing) is of primary importance since it sets your switching parameters. Second would be lekage since this sets how much energy you need to handle in your snubber and peak voltage stress too.


Inductance @ Irated would be the primary magnitizing inductance.
Irated is more problematic, but with 18 ohm on the secondary, I would not like to run 1.5 A though it. Assume primary.
Leakage Inductance. When I model it for simulation, I usually place it on the primary for flyback and secondary otherwise, but that's just me. I would assume primary since 0.75/18 (leakage/magnitizing) gives a resonable ratio (4.2%) for what you end up with for a high turns ratio transformer of this size.


When ever you are in doubt, pleask ask the manufaturer. Asking them to update the datasheet would help others too.