Theres a website with the definition of these all, ithink its a amp manufaterer,but they have good definitions of what everything is there, also theres athread on here with definitions of car audio things such as what you are pertaining to
A subsonic is simply like a super low filter. When you have a ported enclosure, low notes can cause your drive to over exzurt(sp?) itself, causing serious supension damage, and possibly bashing the former on the pole piece, intern ruining your speaker. You set it about 5Hz below tuning, and then don't have to worry about anything.
Some manufacturers may use sub and infra together, so let us knwo what manufacturer you're looking at.
i own a punch 6001 bd amp and it has a infrasonic filter... birthsheet say 705 w rms..... my old rf amp had a subsonic,,,,,, just wanna run one type r 15 in a truck..... whether i need to run it sealed or ported.... i herd that u need a sub son for port...
also called a "rumble filter" because vinyl recordings could impart a low frequency signal from the physical turning rate. this filter blocks subsonic bass (bass that cannot be heard but can be felt). for a woofer in a ported box, this filter can prevent the woofer from moving too far.
some amps have started offering variable Q subsonic filters. this allows the filter to have a bass boost @ 30hz or so, then cut out sounds below 20hz. i'm not a big fan of this, but understand how it works.