If you look up the definitions of tentacles, you will find that they incorporate both the idea of reaching out and grabbing as well as locomotion and exploring. However, in the case of cephalopods we consider appendages of locomotion and exploring different from appendages that are solely used to capture prey. I am not sure if there is an official definition but basic knowledge of the anatomy of the decapods vs octopods reveals a distinct difference in both function and physical makeup between arms and tentacles.
Visually, we make the distinction that arms have suckers along the whole length where tentacles are either suckerless or end in a suckered club. So visually, squid and cuttlefish have 8 arms and two tentacles, octopuses have 8 arms and nautilus have only tentacles.
Functionally, squid and cuttles have two specialized arms used solely to capture prey (or is that 8 specialized tentacles
). The two tentacles can be launched at high speed and clasped together, something the arms can't do. The 8 arms are used for locomotion and manipulation of food once it has been brought to the the mouth area. Octopuses have only 8 appendages that all appear and are used generally in the same manner. Instead of tentacles to catch moving prey, they pounce and trap their meal, again using their arms to hold and manipulate the captured food (there is some speculation that the front 6 are more arm like and the back 2 more leg like in use.
This thought was a recent outcome of a citizen-scientist observation project that was initiated to see if it was worth studying left and right favoring. The mass observations pretty well nixed any future study on the right/left armed thought but suggested the front/back useage differences. You will note that the summary article says "tentacles"
). There probably isn't anything high speed about the nautilus but it does use its tentacles for snaring food and only marginally for moving about.
Soooo ... octopuses vs octopi vs octopodes or arms vs tentacles vs legs all are probably technically acceptable but there is a preferred usage in both cases and our ectrentic interest allows some latitude for snobbery