Limit Burst is a unit's "Ult" in wotvffbe. Only UR units and Mont Leonis have them at this time.
Activating a Limit Burst requires AP.
Basically, melee attackers have a lower chance of using Limit Bursts compared to spell casters.
The high initial AP of spell casters allow them to use Limit Bursts easily.
For more about AP, see here.
A few UR units have very good Limit Bursts such as Ayaka and Mediena. Tap their icon to see their Limit Burst details.
Limit Bursts can be enhanced by consuming Burst Pots.
Burst Pots/Mini Burst Pots are a somewhat rare item because the main way of obtaining them is farming the daily Pot Chamber.
UR Units & Tier List
MR Units & MR Tier List
Upcoming Unit/Vision Card Release Schedule
Warrior of Light | Unit
Kilphe | Unit