Understanding Rp / Rd switch in USB-C DRP/DRD 2.0
The current sources do the same as pullup resistors.  They source a
current into Rd on the other end of the cable.  A current source is used
instead of a pullup resistor probably because accurate current sources
are easier to make on a chip.
When this device is a power source, the other end of the cable will be a
power sink.  It has Rd's.