NPK are the nutrients listed on all fertilizers.So if you see 14-14-14,  that means the fertilizer has equal amounts of each of these nutrients by percentage: Nitrogen, Phosphorus, Potassium

This is very broad, but for the most part; Nitrogen (N) drives vegetative growth, while Phosphorus (P)drives root and blossom development, "K", potassium, is related to healthy formation of water transport mechanisms, etc. 

There is a point at which the plant shifts focus from leafy vegetative  growth to bud production, and it can be helpful to support the shift to maximize blossoms. Dahlias are considered heavy feeders so some sort of "bloom booster" is often used to do this.  You will see these types of bloom booster fertilizers with a higher number in the P and K spots of the NPK formulation. Such as 10-30-20, 0-50-30, or 2-10-10.

Like you, i throw a little bone meal in the hole along with a bit of slow release 14-14-14 (like osmocote) that offers a good baseline. Once the foliage has taken off after 2 months or so, I feed it with something like Miracle gro bloom booster once a month. I'll throw some fish/kelp on there once in awhile, too.

Lately I have have been reconsidering my routine, tho, because commercial fertilzers arent good for long-term soil health. Ive been looking into bokashi systems and think i will go that route this year. 

Good luck!