Intumescent sealants swell when heated and are used to close the small gaps that fire and smoke would otherwise travel through: around door frames, at service penetrations, at the perimeter of a fire resisting wall. They work only in the configuration they were tested in, which is why the tested detail rather than the tube is what you are actually buying.
What they are for
Sealing linear gaps and small penetrations in fire resisting construction. The perimeter joint between a door frame and the wall. The gap where a wall meets a soffit. A small hole where a cable passes through. In each case the sealant restores the fire resistance of the element it is repairing, provided it is used in a detail matching a tested one.
The tested detail is the product
A sealant is tested in a specific construction, at a specific gap width, to a specific depth, with or without backing material. Applied to a wider gap, a thinner bead, or a different substrate it has not been tested and nobody can say what it does. The manufacturer's field of application document is the thing to ask for, and a contractor who cannot produce one for the detail in front of them is guessing.
Sealant, filler and foam are not interchangeable
Ordinary expanding foam is not fire resisting and has no place in fire separating construction, though it is found there constantly because it is quick. Fire rated foams exist and are tested for specific gap sizes. Fire rated mastics handle small movement joints. Fire rated board and batt handle larger openings. Using the quick product because the correct one needs more work is the commonest defect in passive fire protection.
Acoustic and fire performance together
Many products are sold as acoustic and intumescent, because the same joints frequently need both. The two performances are tested separately and a product meeting one does not automatically meet the other. Read which standards the product was tested to and for which construction, rather than assuming the combined description covers your case.
Questions people ask about intumescent sealant
Can I use ordinary expanding foam around a fire door frame?
No. Standard expanding foam is not fire resisting. Fire rated foams and sealants exist and are tested for specific gap sizes and constructions, and the tested detail is what makes them work.
What is a field of application document?
The manufacturer's statement of the constructions, gap sizes and depths a tested product may be used in. It is what tells you whether your detail is covered by the test evidence.
Is intumescent sealant the same as fire rated mastic?
They overlap. Some products intumesce, others are simply fire resisting. What matters is the test evidence for the joint you are sealing rather than the marketing word on the tube.