P7: LED Matrix (Column 8)
P7 is wired to Column 8 of the onboard 5x5 pixel LED matrix on the BBC micro:bit.
While the LED matrix is 5x5 (25) pixels, there are 9 columns and 3 rows. Columns 8 and 9 drive only two LEDs each.
While you may use this pin for other purposes, you must take into account that it is wired to LEDs and their associated resistors.