Apple has recommenced its beta testing cycle following yesterday's release, with new versions of iOS 12.1.1, tvOS 12.1.1, and macOS 10.14.2 available for testing by developers.
Participants in Apple's testing program can acquire the new builds via an over-the-air update on devices already running earlier versions, or can download them from the Apple Developer Center. A public beta version typically arrives shortly after the developer version, and can be picked up from the Apple Beta Software Program website.
So far there are three operating system updates being tested, covering iOS, tvOS, and macOS, with only watchOS missing from the list.
The first iOS 12.1.1 beta has build number 16C5036c. The first tvOS 12.1.1 beta is build 16K5035b, and the first macOS Mojave 10.14.2 beta build is number 18C31g.
At this stage, nothing is known about the contents of each beta. Release notes for each version, which typically provide such information, are unpopulated, but should be updated with information later on Wednesday.
The numbering system for the latest betas suggest it is a minor incremental update in each case, most likely indicating they make small changes for bug fixes and performance improvements, rather than introducing new features.
AppleInsider, and Apple itself, strongly advise beta users to keep from installing the software on "mission-critical" hardware, such as primary devices, due to the potential for data loss, among other possible issues. It is recommended that secondary or non-essential hardware gets used for beta testing, and that regular backups of any important data are performed.
Update: Apple has temporarily pulled the macOS Mojave 10.14.2 beta from its developer website. Why the software was taken down, and when it will return, is unknown at this time.