It is an scheduling algorithm. The Proportional Fairness scheduling rule, as the name suggests, attempts a “proportionally fair” allocation of radio resources to different users.
In context of Carrier Aggregation, I think this refers to taking CC's (component carriers) into account in the eNodeB scheduling algorithm.
As far as the 3GPP specifications are concerned, the scheduling algorithms are left to the eNodeB implementation.
For more details i suggest you to go through this paper.