The MEI Namespace is http://www.music-encoding.org/ns/mei
A PDF version of the MEI 5.0 Guidelines is available here: https://music-encoding.org/guidelines/v5/MEI_Guidelines_v5.0.pdf
New customization: MEI Basic. It provides a simplified subset of the MEI framework that reflects the capabilities of most popular “Common Western Music Notation” score-writing applications currently in use. For more details on MEI Basic see the corresponding section in the guidelines.
The MEI Schemas (specific for CMN, mensural, or neumes notation) are available at the following URLs:
It is strongly recommended to use one of the above schemas, unless you are forced to do otherwise for good reasons. When relying on more than one of the above schemas, or checking the general validity of an MEI file, it can be reasonable to use the MEI ALL schema:
In the context of tutorial exercises (or comparable purposes), the MEI ALL (Any Start Element) schema could be used to provide greater flexibility in terms of removing some technical and framework constraints:
Further detailed customization of a schema, allowing to restrict specific elements or attributes, can be done with the MEI Garage profiler: https://meigarage.edirom.de/profiler.
A detailed overview of the changes for the MEI 5.0 schema is available here: https://music-encoding.org/archive/comparison-5.0.html