The 'Other' payment method allows you to record your sales that use another POS device (like a square device, for instance). This is helpful if you prefer to have one record for all sales of your business in Momence.
Here's how to ring someone up using the Other option for payment method.
You can add an 'Other' (aka Custom) payment method for future use, by clicking Settings > Customer Payment Methods.
You can also add one right from a sale in POS.
After selecting any customer and adding any item to the cart, in the Payment section, click Other > Manage Payment Methods.