From what I've understood it's just the normal renown, but every one of your active units "takes up" some renown, which limits what and how many units you can recruit in your party. Basically imagine this (numbers made up) : you have 150 renown. You get a Chaos Warrior who costs 100 renown. You still have your 150 renown, they're not "spent" - it just means that your available pool for recruiting other units is only 50 renown now, so you can't hire a second Chaos Warrior until you get 50 more renown.
The tier system means that low units cost basically nothing (Marauders are 5 renown iirc ?) and higher-tier units which are insanely stronger (check out the planned stats for the Knights of Nurgle... not sure if Empire stats have been posted or not) are super expensive, but are worth it.