Depending on the shared hosting plan level, your account is limited in several aspects at different levels.

Limits on domains (virtual servers), databases, and disk space are hard, which means that Virtualmin will not let you create more virtual servers or databases and the file system will refuse to store more diskspace than what your limit is. The limit on bandwidth is soft, which means that your site will not be shut down and you will not be charged more if you go over the limit (we do monitor for significant abuses though).

These caps are hard, which means that the operating system will not let you use more memory (per account memory limit applies to RSS, per process memory applies to total virtual memory, i.e. swap) or spawn more processes. The process limit is implemented on LWP level, and applies to lightweight processes or threads actually (may be triggered by a single thread-heavy process too).