bits.wikimedia.org
bits.wikimedia.org is a cookieless domain dedicated for requests to the load.php endpoint of MediaWiki handled by ResourceLoader, and certain static asset requests.
phab:T95448 is working on removing use of this hostname in favour of accessing load.php via the wiki domains.
Bits cluster
bits.wikimedia.org used to be served by the bits cluster (a dedicated pool of Apache servers). Since 2014 this separate cluster no longer exists, the requests are now handled by the main app servers. This improves redundancy/spike proofing.