For purchases (in-app purchases) made through the Mac App Store (MAS), authorization is based on the Apple account used for purchase.
For purchases made through the MAS channel, the same account can be used on multiple devices. To activate on non-purchase machines, go to Menu: Picsee - Settings - Account - Restore Purchases.
For non-MAS channel purchases, one activation code (authorization code) can only activate two devices (MAC) simultaneously.
For non-MAS channel purchases, to change devices, go to Menu: Picsee - Settings - Account - Deactivate This Device to deactivate.