What's the segwit transaction serialization "flag" field for? According to bip 141, the segwit transaction serialization format (used to compute the wtxid) is:

[nVersion][marker][flag][txins][txouts][witness][nLockTime]


The bip mentions that the flag field MUST currently be 0x01. What's the purpose of this field? Is it some kind of upgrading mechanism of the serialization format?
Found it in bip 144:


  Rationale for the 0x01 flag byte in between: this will allow us to easily add more extra non-committed data to transactions (like txouts being spent, ...). It can be interpreted as a bitvector.


So the 0x01 flag is indicating that the witness data should be included in the (bip144) transaction serialization.