Difference between chain.pem and fullchain.pem?

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1

Which one should be used? chain.pem or fullchain.pem?

2

fullchain.pem = cert.pem + chain.pem

You typically use chain.pem (or the first certificate in it) when you're asked for a CA bundle or CA certificate.

You typically use fullchain.pem when you're asked for the entire certificate chain in a single file. For example, this is the case for modern versions of Apache and nginx.

3

Also adding to @griffin reply the fullchain.pem DOES NOT include a CA's Root Certificates.

4

No cert should ever include the signing root certificate.
Any program that requires one, was poorly written.

5

Hear hear.