It could be any of them, some times it could be Conversions or video or even lead gen.
Re-targeting is an audience and the objective is the way of facebook to show your ads to your audience.
The question is what do you want them to do on the re-targeting process, to visit a landing page with a SALE or to sign up to a newsletter
Facebook knows when to display ads to specific users surrounding your objective. If your goal is conversions use it, if you goal is branding use reach, if your goal is traffic use traffic. Basically use what your goal is.
If your audience is under (x) and your retargeting offer is good, isint it true that all of the people in your audience will see the ad(if budget allows) , so the objective becomes more how you can get the cheapest impressions?
f you get a good CPM and your target audience is small enough that you can hit them all with the desire impression count using your budget then just use reach. Use accelerated delivery to make sure that none of your customers get missed because FB is trying to spread out your money