The term 'Alto' is used to signify . . . . . . . . type of cloud.

The term 'Alto' is used to signify a Medium type of cloud.
There are different types of clouds:
High clouds (16,500 - 45,000 feet) Cirrus - these clouds are delicate and feathery that are made mostly of ice crystals.
Mid-level clouds (6,500 - 23,000 feet) Altocumulus - these are small patches of clouds, called cloudlets, which most commonly exist in the shape of rounded clumps.
Low Clouds (Less than 6,500 feet) Cumulus
Special clouds (Contrails)