Watch function (complication)
The moon always faces the earth with the same side. Based upon the fact, that the moon obtains its light from the sun (and at times a very small part from the earth the ashen earthshine which can be observed just before and after new moon), it is changing continuously its face this is called new moon, waxing moon, full moon and waning moon.
A lunar cycle lasts approximately 29.5 days. The result is that it appears at the sky from day to day about 50 minutes later (24 hours divided by 29.5).
A moon-phase indication is a rotating disk, shown in a dial section, that indicates the position of the moon - usually the way it presents itself at the observation of the Northern Hemisphere.
Because of the additional mechanics moon-phase indication is counted as a complication.