Sort:  

It's the default choice when you're replying to comments or submitting a new post i think. You should be able to easily identify it as you see messages about it:

When writing a new post, if if markdown is active you will see the option to switch to the HTML Editor and then the other way around:

Coin Marketplace

STEEM 0.18
TRX 0.16
JST 0.030
BTC 65631.30
ETH 2609.36
USDT 1.00
SBD 2.70