PXT is a format for audio tones, used by the PixTone (not to be confused with PxTone) and SeaTone sound editors. Its intended use is for sound effects, such as for video game development. It was used first in PxTone and then SeaTone was developed to be based on its format and mostly compatible (though some effects can be used in SeaTone that aren't supported in PixTone, so such files wouldn't be compatible).
The format is ASCII text-based, with a series of field names and values (separated by colons) in sections separated by blank lines. It consists of a set of values that represent what waveforms were set up and mixed in the tone editor program.