Provided by Winamp itself (not by user plugins). You can also use these tags with the Toaster plugin.

%filename%: returns the full path of the file. To display the filename only, use the $filepart function.

%folder%: Returns the containing folder. Winamp 5.2+ Only. Provided by Winamp Media Library

%rating%: Returns the track rating as an integer from 1 to 5, if set. To display as stars or another character, use the $repeat function. Winamp 5.2+ Only.

%playcount%: Returns the playcount (number of times track has been played), if the file is stored in the Media Library database. Winamp 5.2+ Only. Provided by Winamp Media Library or input plugin

For the fields listed below, info is taken from ML only if the file is present in the ML database and if the following option is checkmarked:
Prefs (Ctrl+P) > Media Library > Local Media > ‘Use Library title information for Playlist Item Formatting’. In all other cases, the info is provided by the input plugin (if the respective input plugin supports the field). Most of these are straightforward and are taken from the standard file tags, and therefore don’t require any additional explanatory notes.

%artist%: Returns the Artist field.

%title%: Returns the Title field.

%album%: Returns the Album field.

%year%: Returns the Year field.

%genre%: Returns the Genre field.

%comment%: Returns the Comment field.

%tracknumber%: Returns the TrackNumber with no padding, eg. Track 1 will be displayed as 1. To pad with zeros, eg. 01, 02…10, 11, use the $num or $lpad function.

%track%: Same as %tracknumber%

%albumartist%: Returns the Album Artist field. Winamp 5.3+ Only.

%disc%: Returns the Disc No. tag, if available, eg. 1/2 (Disc #1 of a 2CD set).

%composer%: Returns the Composer field.

%publisher%: Returns the Publisher (Record Label) field.
Winamp 5.3+ Only.

%streamtitle%: Returns the Streaming Title for streams, if one exists.

%bitrate%: Returns the Bitrate (will use average bitrate for vbr).

%length%: Returns length of the track in miliseconds. For a formatted track length, use
$div(%length%,60000):$num($div($mod(%length%,60000),1000),2)

%type%: Returns 1 for video, 0 for audio. Winamp 5.2+ Only.

%vbr%: Returns 1 for VBR(Variable Bitrate) audio, 0 for CBR(Constant Bitrate) audio.

%category%: Returns the Category field. Winamp 5.55+ Only.
Provided by input plugin

The fields listed below may not work for all file formats.

%trackartist%: Returns the Track Artist field (equivalent to Artist), if different to Album Artist. Winamp 5.3+ Only.

%mood%: Returns the Mood field. Winamp 5.3+ Only.

%bpm%: Returns the BPM (Beats Per Minute), if data/field exists. Winamp 5.3+ Only.

%key%: Returns the Key field (eg. E Minor), if it exists. Winamp 5.3+ Only.

%lyricist%: Returns the Lyricist field. Winamp 5.3+ Only.

%conductor%: Returns the Conductor field. Winamp 5.3+ Only.

%tool%: Returns the Encoded/Tool (software used to rip/encode track) field, eg. Winamp 5.34. Winamp 5.3+ Only.

%family%: Returns the filetype’s Family, eg. M4A returns: MPEG-4 Audio File Format. Winamp 5.5+ Only.

%encoder%: Returns the Encoder, eg. LAME 3.97. Winamp 5.3+ Only.

%ISRC%: International Standard Recording Code, or ISO 3901 (Info). Winamp 5.3+ Only.

%replaygain_track_gain%: Returns the ReplayGain Track Gain value (eg. -3.16 dB), if it exists. Winamp 5.3+ Only.

%replaygain_album_gain%: Returns the ReplayGain Album Gain value, if it exists. Winamp 5.3+ Only.

%replaygain_track_peak%: Returns the ReplayGain Track Peak value, if it exists. Winamp 5.3+ Only.

%replaygain_album_peak%: Returns the ReplayGain Album Peak value, if it exists. Winamp 5.3+ Only.

%gain%: Returns the Gain value, if it exists. Winamp 5.3+ Only.