A440 tuning is the standard tuning most guitar players tune to. If you look at any electronic tuner you'll usually see something like.....cent -50.....cent +50 and ALTERNATIVELY Hz 430 440 and 450 Hz.
If you tune your guitar to a440 it means standard tuning. If you want to tune down by one note, use the 430 CENT.
A440 is known as as the standard tuning for all guitars.