The rental calendar is used for short stay tenants like holiday homes or holiday parks.  Because of the speed of data entry required for the volume of transactions these people get, the system was designed to be as simple as possible when entering rent income transactions.  So the income groups was fixed at 'Rent' and disabled.

With the next version for short stay tenants there is now the possibility to use other income groups when taking a payment from the tenant, as long as that income groups is marked as an income group used to pay for extra charges.  If rent is used to pay extra charges then there will be no difference.  For example if Linen hire is logged as an extra charge against the tenant, you can now choose to use a different income group to record the payment of the extra charges separate to the rent.  The 2 tone green colour showing percentage paid also takes into account any extra charges charged and paid.

To use this new feature you will need to go to the maintenance menu and choose Extra Charge Income Groups anmd specify which income groups are used to record payment of extra charges.

While I was at it, I also noticed something else.  If you use the Take a Payment button, the calendar screen did not immediately reflect the amount paid using the 2 tone green colour.  It now updates that green colour immediately after entering the income transaction.