pact is short for pattern activity. These files are the generic output files of a PatternMonitor. As other Auryn output formats they are organized as timeseries in columnar structure. The first column contains the time. The following columns contain the population firing rate of a given population as well as the running overlap of the network activity with a given pattern (see pat file) that was loaded into the PatternMonitor. The overlap is essentially the population rate of the concerned pattern. The last column monitors the total population firing rate for reference.
Suppose you are monitoring 10 patterns, you will therefore find 22 columns. 10×2 for population firing rate and relative pattern activity plus 1x time and 1x population firing rate.