Hello Brahim
When configuring the PVC circuit of a DSL connection, you must choose a method of encapsulation for what is known as the ATM adaptation layer or AAL. By default, the aal5snap encapsulation is set, but different types of encapsulation support different features. In particular, the aal5mux option used here specifies an AAL type for multiplexed VCs. When this is used, a protocol must be specified, and in this case, Rene is using ppp.
You can find out detailed information about the various options for these configurations at the following Cisco command-line reference documentation.
I hope this has been helpful!
Laz