Safe bet its for a dim module. Type though is , like they said, really only determinable by seeing the ballasts or knowing the light manuf and some research.
more likely.....2-WireDimmed Input
These are very common ballasts and the easiest to install. They require a dimmed hot and a neutral (ground is understood) and are available in 5% dimmed models from such companies as Lutronand Advance (Philips). They are installed and controlled on one dimmer just as you would an incandescent source except that a bottom-end threshold is set. This setting keeps the lamps from running below their recommended voltage, preventing premature failure of both lamps and ballasts.
2-wire ballasts are made in both forward phase and reverse phase variants. To dim a reverse phase ballast you will need to use a reverse phase dimmer module such as ETC's ELV10 dimmer in a compatible dimmer rack.