I suspect it's either a seal to stop someone fiddling with the spring set points or a seal which breaks if the valve ever lifts.

Can you post a diagram?

Resilient seat seal means a resilient seal on the working seal, I.e. not metal to metal so only suitable for certain fluid and temperature

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LittleInch got in one (or two).
The 'Lead Seal' is always affixed to the outside of a Pressure-relief Valve as a visible means to show if the valve have been tampered with ie., cap removed to alter the set pressure. It is normally done by the manufacture and/or the last person/company that tested the valve - it should be embossed with some ID supported by a test certificate - Some end users are steering away from lead but the term 'lead seal' is still used.

'Resilient Seat Seal' - Relates to the Pressure-relief Valves seat if fitted with an O-Ring (as opposed to a metal to metal seat).

*** Per ISO-4126, the generic term
'Safety Valve' is used regardless of application or design ***

*** 'Pressure-relief Valve' is the equivalent ASME/API term ***

That strange little grey thing hanging on the thin wire is the "Lead Seal"

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Also: If you get a response it's polite to respond to it.

Thank you all for taking the time to reply to my question. LittleInch thanks for the image you posted, it really helped me to differentiate between the two.

No problem.

It's good to know we helped - so many posters just vanish and never let us know....

You often see a plastic tag nowadays doing the same thing - same sort of thing you often see on fire extinguishers to show that the pin has been pulled.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.