From the AoS Facebook feed....

Quote Originally Posted by Aos Facebook
So, what's a Battleline unit?

We're glad you asked!

This is something from the upcoming General's Handbook.

Our gamers are building a force they can use in any style of game, and when playing the more competitive Matched Play format, your army must include a minimum number of Battleline units, depending on the size of the game. (We'll have more on how this works, and the other types of units soon...)

The Crypt Flayers and Brutes are actually not Battleline units, unless the whole force has the Flesh-Eater Courts and Ironjawz keywords respectively, but this is what both generals plan to do. This is one of the advantages in Matched Play in sticking within a single themed force
So definitely channeling the more traditional setup we're used to. Definitely intrigued, and yes, I would like to know more!