Noob question - Choosing Units from DE Codex vs Covens sup

Hi all,

My Question is:

if I choose to take a unit of grots, do I decide if they come from the supplement or from the DE Codex? And if its really a choice, why wouldn't you choose the Covens supplement?

As each unit has to be part of a detachment, in effect you do have to decided where they come from.

The main reason you would take them as DE is that the cost to run them as covens (either in a grotesquerie or the main coven detachment) can be quite high and hard to model

Minimum cost for a unit of grots from the codex: 105. Minimum cost for a Grotesquerie: 280.

Some formats will also limit your number of detachments.

Thanks all. That sums it up nicely.