PSFXL files are mainly generated by the Cadence Spectre simulator. The format is an extension of the PSF file format. It allows higher read/write speeds. Therefore Spectre generates PSFXL files for transient simulations.
In addition to the data files Spectre writes a “logFile” which lists all files containing simulation data. This post gives more details on the “logFile” and the PSF and PSFXL file structure.
BeSpice Wave and the underlying waveform parser read PSFXL files. The curve data is loaded dynamically when required. It can be parsed while running a simulation. BeSpice Wave automatically detects when the file changes on disk and reloads it.