From Black Mesa: Wiki

Semi-protection prevents edits from any account that is not autoconfirmed (newly created account or has not contributed enough). Such users can request edits to a semi-protected page by proposing them on its talk page. Administrators may apply indefinite semi-protection to pages which are subject to heavy and persistent vandalism or violations of content policy. Specific reasons include:

• Subject to vandalism or disruption.
• Subject to edit-warring where all parties involved are new editors (i.e., in cases in which full-protection would otherwise be applied). This does not apply when autoconfirmed users are involved.
• Article discussion pages, when they have been subject to persistent disruption. Such protection should be used sparingly because it prevents newly registered users from participating in discussions. A page and its talk page should not both be protected at the same time.
• Talk pages of blocked IP addresses that are being used for continued inappropriate editing, including continued uncivil or offensive remarks. The protection should be timed so as to not exceed the length of the block. Although administrators may also choose to change the block settings to block the user from editing their talk page instead, since there is no need to synchronize the block period with the page protection time period with this method.