If you are in a colocation facility, it is likely that there is a large ATS "upstream" from you, connecting two different power sources to your circuit. Of course, that doesn't protect you from a problem on your particular circuit, or between the power panel serving you and the ATS.
If you go for a small (let's call anything under 50A per leg small) ATS for your rack, there are a number of "gotcha's" to watch out for.
1) The big one - make sure both inputs to your ATS are fed from the same electrical phase. It will make your life substantially less miserable. ATS vendors like to talk about "zero transfer time" and "break before make" without noting that those two are mutually exclusive. And since relays are mechanical, there is some variation between units (or between relays in the same unit). You can get "relay weld" where your ATS becomes a short circuit between two different phases and your ATS will lose, likely tripping either one or both facility circuit breakers. You can usually get all your facility power on the same phase if you specify that during purchasing. Getting it fixed later is likely to be expensive as it involves at least some re-engineering of your power feed (usually just moving one breaker up or down, but they still charge a lot). My standard spec is "Qty 2 L5-30 receptacles per bay, fed from different panels and different PDUs (and ideally from different UPSs) with no part of the physical path in common." You'll want to specify the type of receptacles you need, L5-30 is what I use.
2) Many of these ATS units have customizable sensitivity and response time. You'll want to select the settings that make it least likely to change sources, as long as your equipment is still happy. Most modern equipment has enough hold-up time in its power supplies that you can use the least sensitive settings. In most cases, you'll also want to configure the unit as "non-reverting" (or, as some brands describe it, "preferred source: none"). Otherwise you can get unnecessary transfers back when a transient power problem clears up. The only time you wouldn't want to do this is when you have oddball billing for electrical usage, where you're supposed to always be on one circuit except during an actual ATS event.
3) Keep a careful eye on your total current consumption. If you have a pair of 20A circuits and have loaded both of them to a total of more than 20A, an ATS event that moves all of your loads from both circuits onto a single circuit will cause you to overload that circuit. This only happens if you have equipment that not connected to that ATS plugged into the circuits that feed the ATS (anything plugged into the ATS will be on either the A feed or the B feed, depending on which the ATS selects.
