This is just a fun off-meta role that works quite well in my experience. This is also my first guide and probably my last unless i get some sort of recognition in comment/likes.

What Cassiopeia APC brings to the table:
- Very high damage early on (more than most ADCs)
- Large amount of CC unlike most ADCs
- Great kite and chase potential
- Great in team fights with her AoE/DoT/CC and single target burst
- Good sustain right from the start of the game
- Can stop mobile ADCs from flashing or dashing (Lucian, Tristana, Vayne, etc)
- Very easy farm with high damage and practically free E's

Why can this be better than Cassiopeia Mid/Top?
Most mid and top laners have the potential to outduel you and have high burst, both of which are a problem for Cassiopeia. In bot lane you have supports who can peel and lock down enemies for you, and since most ADCs rely on consistent damage you should be able to destroy them with your much stronger consistent damage.

Farming Combo: AA -> E (gives quick burst and refunds mana + a bit extra due to Doran's)

Post-Rylai's Easy Poison: E (for a cheap/quick slow) -> Q -> EEE

Due to the minimum range don't bother using W when running away unless the enemy is quite a distance away from you.

Cassiopeia APC is very strong and I'm sure if you decide to play Cassiopeia this way you will have a lot of fun. Great to do as a premade bot lane because some supports might be slightly unnerved at an off-meta ally.

This guide is mainly just about the APC potential for Cassiopeia so if you want more tips and other in-depth information about Cassiopeia it would probably be best to look at another guide in addition to this one.