Just been a lurker now and then around here, but then i see this and no one here seem to actually know how it works, i can't let this misinformation go uncorrected. Switch Axe's phials/coating/vials, whichever you wanna call it is not the same as bow's coating at all.

Each switch axe's phial is FIXED, just like how a gunlance's shell type i.e. normal/long/spread types is FIXED for each gunlance. You don't go loading and changing gunlance's shells with bowgun bullets now do you?

The phial's effects only come into play when in sword mode, and your gauge depletes as you use sword mode. The gauge auto regens in axe mode and if it extremely low it says RELOAD you can press R in axe mode that will refill the gauge instead of switching to sword mode.

There's 6 types of phials:
power (boosts the raw)
element (boosts sa's innate element and status effect values)
poison, para, exhaust(deals ko if hit head and fatigues boss and makes em goes drool status like hammer), dragon(adds some dragon element)

Now for example the difference between SA with innate para attribute that has element phial and raw SA that has para phial, for the former you'll be doing para status attacks in both axe and sword mode but the para value is boosted more when in sword mode. For the latter tho, you'll only be doing raw damage when in axe and does para in sword.

In general, an elemental SA will either comes with power or element phial, status ones comes with element phial and raw ones can come with any one of em cept for element.

A noteworthy early SA, the yukumo lookalike one that is made from red ticket given to you by the elder, it comes with very long green bar and good raw and has dragon coating. It's probably one of the easiest and earliest way to get some dragon element weapon for dragon weak stuff.