AcqKnowledge files are saved by the BioPac system to save time series of physiological measurements -- for example, blood pressure, blood oxygenation, temperature, or skin conductance. It is a multi-channel file format in which each channel can have its own sampling rate (expressed relative to the file's master sampling rate). In addition, each channel may be integer or floating-point in nature.
There are two main flavors of AcqKnowledge files (both having the .acq extension): uncompressed and compressed. In the uncompressed file format, channel data is interleaved together; in the compressed format, each channel is stored as its own array, compressed by zlib.
The format is partially documented in BioPac's AppNote 156.
BioPac's own AcqKnowledge software can read and write .acq files. In addition, Bioread is a Python-based open-source library that reads all known versions of .ACQ files. There is also a Matlab-based library to do the same thing.