TXT was originally meant for human-readable notes. Today it carries SPF policies, DKIM public keys, DMARC policies, ACME challenges and verification tokens for Microsoft 365, Google and other services.
A single string inside a TXT record is limited to 255 characters. Longer values such as 2048-bit DKIM keys are split into several strings that the receiver concatenates. Some DNS panels split automatically, others do not.
Frequent mistake: two separate SPF TXT records on the same domain, which makes SPF fail with permerror. Extra quotes that a web interface stores as part of the value cause similar trouble.
Example
dig +short TXT example.com