As per Kore's comment on: https://github.com/ezsystems/ezpublish-kernel/pull/436#issuecomment-20733780:

Naming

Please rename "ez_mid" and "ez_mstring" to "ez_multi_id" and "ez_multi_string" in eZ/Publish/Core/Persistence/Solr/Content/Search/FieldNameGenerator.php and the respective search field types. Maybe even "ez_id_array" / "ez_string_array" would even be more readable.

Those identifiers are supposed to be readable by implementers of the Persistence API and they should provide a basic insight of their meaning. Abbreviations should always be avoided. The internal identifier in Solr is something different, since it not part of the external API. "ms" is fine there.