I believe they are trying to say you must use both primary windings.
I would have added: "To get the rated output current"
In other words, if you want to use the transformer with a 115 volt input, you cannot use just 1 and 2 or 5 and 6.
Yes, just this. I am lost as to why they would add the secondaries in this statement. It only makes sense for the primaries.
This makes the most sense! But man, they could have said that better.
Agreed. They could have simply left the secondaries out of the statement for it to become less confusing - and then added my suggested comment for it to be pretty clear.
Then again, the work that's gone into the engineering of a product is usually more thorough than the documentation. The only thing I can say is that following the instruction as written will result in proper operation of the transformer.
I think they are trying to say that their 4kV isolation guarantee does not exist between the two secondaries, or between the two primaries. So, on the secondary, +12/-12 would be fine; they have the same common, but maybe not ok to generate two totally separate outputs expected to be completely independent.
While I tend to agree with your conclusion, I'm not too sure they are being explicit enough in the datasheet. In the statement on Safety, they say:
"These units are designed with 4000VAC isolation between the primary and secondary, and also, between each winding and the core." which leaves as much unsaid as said. In some applications, knowing the rated isolation between the two secondaries would be useful. The isolation between the two primaries - not so much.
While this datasheet will provide sufficient information for
typical applications, it's a bit lacking as far as providing full engineering specifications - IMHO.