The location and type of attachment required depends on the actual movement of tooth required. They should be placed into a location where a "pushing" action will deliver the correct movement since aligners are unable to "pull" teeth in any direction as braces can.
Other attachments are used as anchors and these will need to be placed in a location to resist forces of displacement. They should also be put in a location where they will not be broken off when the teeth are closed together.
At times, attachments are placed on the inside of the upper teeth. These may be used as "bite turbos" to help open the bite or torque the teeth. Again, depending on the movement required, attachments on the lingual surface may work as well or better as those on the front.
To keep attachments less visible, more experienced providers are now abandoning the ellipspoid (oval) attachments for a rectangular bevelled one near the incisal edge (biting) surface of the tooth.