TJA is a file format created for Taiko simulators to have playable charts. It contains the metadata and the notation for all of the song's difficulty levels. "WAVE:" in the file points to an external audio file that should be in the same directory as the TJA.
This page covers TJA support in taiko-web, but may apply to other simulators as well.
TJA file is a plain text file that can be encoded as either UTF-8 with BOM or as Shift-JIS. The extension is ".tja". The container format for the audio file does not matter as long as it is supported in the simulator. If the TJA file is hosted on taiko-web, the filename should be "main.tja" for the TJA file and "main.ogg" for the audio file.
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