Enhancing standard datasource, small clarification.

I have gone thru Robert's blog it says

*"One method of adding user-defined fields is the following: add the required fields to the communication structures (MCVBAK, MCVBAP and so on ) using the append method (via SE11) and then use the LIS customer exits to fill the field.

For information on enhancing the communication structures, you can see the documentation for the enhancements MCS10001, MCS50001 and MCS60001 provided in transaction SMOD.

After you enhance the communication structures you can then enhance the extract structure with the relevant field in the customizing cockpit (transaction LBWE), provided that the communication structure is available in the selection. Then you can proceed with the steps described before in the previous bullet."*

See if in one DS 2LIS_11_VASCL he want to add a field AUDAT and my doubt is how can he add that field to the communication structure so that he can select it from that DS communication structure to the left side, Is it possible can we add what ever fields to the communication structure. I thought we should append the datasource and then enhance the respective function exit rite. Why are we talking about append method via SE11, can anyone please guide what do we mean by appending it throug se11 to the communication structure. What i know is if we do not find the required filed in any standard ds we can append the ds and edit the related include program or use 2 DS's to get the required fields to a single cube on BI side after activating those fields if we know those field names. What is the concept of adding fields to the communication strucute using append method via SE11. Please guide me in detail. Thanks in advance.