Phantom Center is a stereo imager plugin that works by extracting the phantom center from a stereo signal (this is not mid/side).
Contrary to your usual mid/side processor, the sides components are here a lot more decorrelated.
The HPF and LPF parameters control the cutoff frequencies of the filters applied to the extracted phantom center. What’s outside this frequency range is redirected to the side channels.
It’s also possible to get separated LCR channels if you want to process them individually (see the user guide). Note that the extraction algorithm is more on the aggressive side and wasn’t specifically designed for “upmixing”. However, just splitting and recombining is lossless.