Hello Janka,
Using the L2Cap layer for communicating with the device implies a greater understanding of BLE than those using the GATT or HCI. You should be reading the Core Bluetooth v4.0 guide section that covers the L2CAP. Most, if not all the functions in the source code would have to conform to the structure outlined in the specifications.
To my knowledge, there isn't any documentation as in a PDF that covers this. You may find information in the BLE HTML that is in the documents folder of some help.
Thanks,