can I use a 10WATT LED BR30 on a recessed light trim that says MAX:9W BR30 LED?
I'm not sure anyone can give you a definitive answer on this.  You could ask the manufacturer whether 10 watts is close enough, but they came up with 9 watts as the maximum; if 10 watts was really the maximum, they would have labelled it 10 watts.  That said, I'll offer a guess.

I can't envision a 110V fixture where a 10 watt load would overheat the wiring or electrical elements, or damage the fixture.  My guess is that the rating relates to the LED bulb.  LED bulbs need to dissipate the heat they produce in order to keep the electronics in a working temperature range.  If you exceed the maximum temperature range for the bulb, the bulb will have a much shorter life.

Recessed fixtures trap heat.  The 9 watts probably reflects the amount of heat it can get rid of to keep up with what the bulb produces.  A higher wattage bulb will generate heat faster than the fixture can dissipate, so the temperature inside the fixture will continue to rise.  Eventually, it will exceed the maximum temperature the bulb is designed to operate in, and the bulb will have a short life.