PWT and PWL relation

Hi, I've playing for a while with the buzzer & PWT registers and I'm able to hear 'sound' according to the frequencies supported. The thing I can't not understand well is the relation between the PWT (pulse width tones) and the PWL (pulse width light), if exists: A) After loading some frequency into the PWT, set the volume and hear the it, if I change PWT to BU in the ASIC_CONF_REG, backlight intensity changes and sound stops. B) With LT output selected in the ASIC_CONF_REG instead of PWL, changes over the PWT registers produce nothing.