The ContentCodes are the available tag types. The list of the available content codes is an mccr tag and can be requested with GET /content-codes.
iTunes (as of 5.0) knows the following ContentCodes:
Code, Name, Type
abal, daap.browsealbumlisting, 0x0C
abar, daap.browseartistlisting, 0x0C
abcp, daap.browsecomposerlisting, 0x0C
abgn, daap.browsegenrelisting, 0x0C
abpl, daap.baseplaylist, 0x01
abro, daap.databasebrowse, 0x0C
adbs, daap.databasesongs, 0x0C
aeNV, com.apple.itunes.norm-volume, 0x05
aeSP, com.apple.itunes.smart-playlist, 0x01
aply, daap.databaseplaylists, 0x0C
apro, daap.protocolversion, 0x0B
apso, daap.playlistsongs, 0x0C
arif, daap.resolveinfo, 0x0C
arsv, daap.resolve, 0x0C
asal, daap.songalbum, 0x09
asar, daap.songartist, 0x09
asbr, daap.songbitrate, 0x03
asbt, daap.songbeatsperminute, 0x03
ascm, daap.songcomment, 0x09
asco, daap.songcompilation, 0x01
ascp, daap.songcomposer, 0x09
asda, daap.songdateadded, 0x0A
asdb, daap.songdisabled, 0x01
asdc, daap.songdisccount, 0x03
asdk, daap.songdatakind, 0x01
asdm, daap.songdatemodified, 0x0A
asdn, daap.songdiscnumber, 0x03
asdt, daap.songdescription, 0x09
aseq, daap.songeqpreset, 0x09
asfm, daap.songformat, 0x09
asgn, daap.songgenre, 0x09
asrv, daap.songrelativevolume, 0x02
assp, daap.songstoptime, 0x05
assr, daap.songsamplerate, 0x05
asst, daap.songstarttime, 0x05
assz, daap.songsize, 0x05
astc, daap.songtrackcount, 0x03
astm, daap.songtime, 0x05
astn, daap.songtracknumber, 0x03
asul, daap.songdataurl, 0x09
asur, daap.songuserrating, 0x01
asyr, daap.songyear, 0x03
avdb, daap.serverdatabases, 0x0C
mbcl, dmap.bag, 0x0C
mccr, dmap.contentcodesresponse, 0x0C
mcna, dmap.contentcodesname, 0x09
mcnm, dmap.contentcodesnumber, 0x05
mcon, dmap.container, 0x0C
mctc, dmap.containercount, 0x05
mcti, dmap.containeritemid, 0x05
mcty, dmap.contentcodestype, 0x03
mdcl, dmap.dictionary, 0x0C
miid, dmap.itemid, 0x05
mikd, dmap.itemkind, 0x01
mimc, dmap.itemcount, 0x05
minm, dmap.itemname, 0x09
mlcl, dmap.listing, 0x0C
mlid, dmap.sessionid, 0x05
mlit, dmap.listingitem, 0x0C
mlog, dmap.loginresponse, 0x0C
mpco, dmap.parentcontainerid, 0x05
mper, dmap.persistentid, 0x07
mpro, dmap.protocolversion, 0x0B
mrco, dmap.returnedcount, 0x05
msal, dmap.supportsautologout, 0x01
msau, dmap.authenticationmethod, 0x01
msbr, dmap.supportsbrowse, 0x01
msdc, dmap.databasescount, 0x05
msex, dmap.supportsextensions, 0x01
msix, dmap.supportsindex, 0x01
mslr, dmap.loginrequired, 0x01
mspi, dmap.supportspersistentids, 0x01
msqy, dmap.supportsquery, 0x01
msrs, dmap.supportsresolve, 0x01
msrv, dmap.serverinforesponse, 0x0C
mstm, dmap.timeoutinterval, 0x05
msts, dmap.statusstring, 0x09
mstt, dmap.status, 0x05
msup, dmap.supportsupdate, 0x01
mtco, dmap.specifiedtotalcount, 0x05
mudl, dmap.deletedidlisting, 0x0C
mupd, dmap.updateresponse, 0x0C
musr, dmap.serverrevision, 0x05
muty, dmap.updatetype, 0x01
asrv 0x 00 02 (was 0x 00 01) daap.songrelativevolume (new) msas 0x 00 01 dmap.authenticationschemes ascd 0x 00 05 daap.songcodectype ascs 0x 00 05 daap.songcodecsubtype aeSV 0x 00 05 com.apple.itunes.music-sharing-version aePI 0x 00 05 com.apple.itunes.itms-playlistid aeCI 0x 00 05 com.apple.itunes.itms-composerid aeGI 0x 00 05 com.apple.itunes.itms-genreid aeAI 0x 00 05 com.apple.itunes.itms-artistid aeSI 0x 00 05 com.apple.itunes.itms-songid
Type, codename, description
/* iTunes 6.0.2+ */
0x01, "aeHV", "com.apple.itunes.has-video"
/* iTunes 6.0.4+ */
0x05, "msas", "dmap.authenticationschemes"
0x0A, "mstc", "dmap.utctime"
0x06, "msto", "dmap.utcoffset"
0x09, "asct", "daap.songcategory"
0x09, "ascn", "daap.songcontentdescription"
0x09, "aslc", "daap.songlongcontentdescription"
0x09, "asky", "daap.songkeywords"
0x01, "apsm", "daap.playlistshufflemode"
0x01, "aprm", "daap.playlistrepeatmode"
0x01, "aePC", "com.apple.itunes.is-podcast"
0x01, "aePP", "com.apple.itunes.is-podcast-playlist"
0x01, "aeMK", "com.apple.itunes.mediakind"
0x09, "aeSN", "com.apple.itunes.series-name"
0x09, "aeNN", "com.apple.itunes.network-name"
0x09, "aeEN", "com.apple.itunes.episode-num-str"
0x05, "aeES", "com.apple.itunes.episode-sort"
0x05, "aeSU", "com.apple.itunes.season-num"
0x01, "asgp", "daap.songgapless"
0x01, "aePS", "com.apple.itunes.special-playlist"
0x03, "ased", "daap.songextradata"
0x0A, "asdr", "daap.songdatereleased"
0x0A, "asdp", "daap.songdatepurchased"
0x01, "ashp", "daap.songhasbeenplayed"
0x09, "assn", "daap.sortname"
0x09, "assa", "daap.sortartist"
0x09, "assl", "daap.sortalbumartist"
0x09, "assc", "daap.sortalbum"
0x09, "assc", "daap.sortcomposer"
0x09, "asss", "daap.sortseriesname"
0x01, "asbk", "daap.songbookmarkable"
0x05, "asbo", "daap.songbookmark"
0x05, "aeGH", "com.apple.itunes.gapless-heur"
0x09, "asaa", "daap.songalbumartist"
0x05, "aeGD", "com.apple.itunes.gapless-enc-dr"
0x05, "aeGE", "com.apple.itunes.gapless-enc-del"
0x07, "aeGR", "com.apple.itunes.gapless-resy"
0x07, "aeGU", "com.apple.itunes.gapless-dur"
0x09, "aspu", "daap.songpodcasturl"
0x09, "aeCR", "com.apple.itunes.content-rating"
0x07, "asai", "daap.songalbumid"
0x07, "asls", "daap.songlongsize"
0x01, "aeSG", "com.apple.itunes.saved-genius"
0x05, "meds", "dmap.editcommandssupported"
0x01, "aeHD", "com.apple.itunes.is-hd-video"
Type indicates number of bytes in which codename data must be present
Type Size in Bytes 0x0001 1 byte integer 0x0002 1 unsigned byte integer 0x0003 2 byte integer 0x0003 2 (unsigned ?) byte integer 0x0005 4 byte integer 0x0006 4 byte showing seconds (used for dmap.utcoffset) 0x0007 8 byte long integer (persistent id, song album id, song size etc) 0x0009 text string 0x000A date/timestamp 0x000B version (protocol version etc.) 0x000C list/container(can contain other tags)