as far as I know only the following mp4 metadata tags are defined:
| Tag Name | Type |
| Name | string |
| Artist | string |
| Album Artist | string |
| Album | string |
| Grouping | string |
| Composer | string |
| Comments | string |
| Genre | string |
| Release Date | YYYY-MM-DD string |
| Track # | track#/totalTracks# string |
| Disk # | disk#/totalDisks# string |
| TV Show | string |
| TV Episode # | string |
| TV Network | string |
| TV Episode ID | number |
| TV Season | string |
| Description | string |
| Long Description | string |
| Rating | string, see the table below |
| Rating Annotation | string |
| Studio | string |
| Cast | string |
| Director | string |
| Codirector | string |
| Producers | string |
| Screenwriters | string |
| Lyrics | string |
| Copyright | string |
| Encoding Tool | string |
| Encoded By | string |
| contentID | string |
| HD Video | bool |
| Gapless | bool |
| Content Rating | string, see the table below |
| Media Kind | string, see the table below |
| Artwork | string, path to the file |
MCEBuddy supports most of these. If there is something missing let me know.