Why FLC is the spec that has to be right
A thermal overload relay protects a motor by tripping when sustained current exceeds a safe level — but it only does its job if its trip range actually matches the motor it's protecting. That match is expressed as FLC, Full Load Current: read the figure off the motor's nameplate, then select (or, on adjustable units, dial in) a relay whose FLC range brackets it. An undersized range causes nuisance trips on normal starting current; an oversized range — chosen "to be safe" — won't trip in time to actually protect the motor from sustained overload. Neither is a safe substitute for getting the range right.
Pole configuration and contact state
Pole configuration (3-pole for standard three-phase motor circuits, 4-pole where a neutral or additional switched line is involved) has to match both the motor's phase configuration and the contactor the relay pairs with — they're generally not interchangeable. Separately, check "normal state configuration" (for example 1NC, 3NO) against what your control circuit or PLC program expects: this describes which contacts are open or closed when the relay is de-energised, and getting it wrong shows up as inverted run/fault signalling rather than a hardware fault.
Control voltage and auxiliary contacts
The relay coil's control voltage has to match your control circuit voltage — ranges commonly span 24V up to 690V depending on the installation. If the relay also needs to drive an indicator lamp, a PLC input, or another auxiliary device, check the number and type of auxiliary contacts available before ordering, rather than assuming the base unit includes them.