CBL-Mariner is Microsoft's internal tool to create Linux distributions. Meant to power Microsoft's own Cloud infrastructure, CBL-Mariner distributions aim to consume limited disk and memory resources, as well as offer minimal attack surface.
CBL-Mariner has been engineered with the notion that a small common core set of packages can address the universal needs of first party cloud and edge services while allowing individual teams to layer additional packages on top of the common core to produce images for their workloads.
Leveraging the RPM package manager, the CBL-Mariner build system is able to generate a set of required packages based on a provided SPEC file. Those packages can then be assembled into an installable image. CBL-Mariner also uses RPM-OSTree to support an image based update model that can be used for servicing and rollback in an atomic way.
Microsoft describes two ways to use CBL-Mariner. The fastest way is to build one of the pre-defined images. This process is quick and has no intricacies. Microsoft, though, only recommends it for trying out the distribution, with the preferred approach being building a custom image including only required packages.
Usually it is best to work in a smaller, problem focused environment where you can quickly build just what you need, and rely on the fact that the curated CBL-Mariner packages are already available in the cloud. In this way, you can customize an image with your preferred disk layout or adding supplemental packages that CBL-Mariner may not provide.
Microsoft provides a complete description of all the steps you need to carry through to create a custom distribution using CBL-Mariner.
CBL-Mariner is in some ways related to the Photon OS Project and to Fedora, from both of which it leverages a number of SPEC files. CBL-Mariner is mostly licensed under the MIT license, with Linux contributions licensed under GPL2 and drivers coming with their own licenses.