It means that anything over that limit will be rejected with a 500-Internal Server Error code.
To change what it does when it hits that error the SecResponseBodyLimitAction
directive affects that.
By default, ModSecurity will reject a response body that is longer than specified. Some web sites, however, will produce very long responses, making it difficult to come up with a reasonable limit.
Reject
causes the 500 error, where ProcessPartial
allows even long responses to be allowed, but only the first part of long responses will be processed against mod_security.
https://lists.owasp.org/pipermail/owasp-modsecurity-core-rule-set/2011-April/000694.html