what does your return will be transmitted or postmarked before October 15th mean?

[kekej4321]

how would I know?

[SteamTrain]

IF you are preparing an "Extension" in order to not have to file by 15 April.

...then it is up to YOU to make sure your final tax return is filed before 15 October...or penalties and interest may be assessed.  If you expect to owe some more taxes when you do file, then you are also required to pay the amount you expect to owe, at the time you file your extension paperwork.