The MM protocol is limited to five functions per address. But mfx decoders that also support the MM protocol are not limited to one MM address only, allowing you to control 9, 13, 16, or even 17 functions.
The first mfx decoders that were used in Märklin locos were made by ESU. And as far as I know all mfx decoders by ESU support two MM addresses and allow using 9 functions with MM protocol.
Later Märklin started using mfx decoders they made themselves (beginning 2009). These decoders also support two MM addresses and allow up to 9 functions to be controlled with MM protocol.
With current mfx decoders you can use one, two, three, or even four MM addresses to control up to 16 functions.
Older mfx Decoders
Older mfx decoders have a main address (for speed, lights (F0) and F1 through F4) that can be set via register 1. The secondary address (for F5 through F8) can be set via register 75.
New Decoders
With new decoders you can use register 49 to select how many addresses can be used with MM protocol. Values that worked for me range from four (for only one address) to seven (for four addresses).
Just like with the older decoders, you set the first address via register 1 and the second address via register 75. Register 17 sets the third address (for F9 through F12). And register 18 can be used to set the fourth address (for F13 through F16).
Note: With some factory-installed mfx decoders in Märklin locos I was not able to activate four addresses or to set addresses 3 and 4 using a CU 6021. After activating and setting those addresses with a Central Station I was able to use them with a CU 6021. So to use four addresses with factory-installed decoders with a CU 6021, you may need one-time help from a person with a Central Station.
Using Locos with More Than One Address
Speed knob and function/off will only have an effect with the first address of a decoder. With further addresses only F1 through F4 will work. With the second address, the functions F1 through F4 will switch decoder functions F5 through F8. Similarly the third address will control decoder functions F9 through F12 and the fourth address will control F13 through F16.
Tip for Control Unit 6021
With the CU 6021 you can use the L key to set the loco address – the address is used for the speed knob and function/off. With the F key you select the address that will be used for F1 through F4.
By default F and L use the same address and when a new address is selected, it will be used for F and L automatically (in this mode the lights at F and L are both on).
Simply press F and enter a new address. Now you will either see a light at the F key or the L key. Just press either the L or F key to see the current address or enter a new address. And to get back to default mode, simply select the same address for both – both lights at L and F will be on again and any new address will be used for both L and F.
Another Tip for CU 6021 Users
With four addresses per mfx loco, only 20 mfx locos are enough to fill the address range of the CU 6021 (80 addresses) completely. But in certain cases it is possible to use one address for more than one loco with no or little interference. Additional addresses of mfx locos only use F1 through F4. Early Digital locos or Delta locos do not support F1 through F4 and many fx locos only support F4.
Therefore it is possible to use e.g. address 17 for an early Digital or Delta loco while at the same time using addresses 16 and 17 for an mfx loco.