One of the biggest changes put in place by the 2011 CBA was the “Total Rookie Compensation Pool” which pretty much totally revamped how draft picks are paid and replaced the old “Rookie Salary Cap”. The new system limits not only the first year pay for rookies, but also the total compensation that can be paid to the player over the life of the player’s contract.
Under this new system, the rookie salaries are basically slotted, which, as evidenced by the much earlier signings of many draft picks, means that draft picks are much easier to sign.
This slotting system is also how the league determines a team’s Rookie pool. Each team’s pool is determined by the number of draft picks and where those picks were chosen. From year to year, it is likely that those numbers will increase proportionately to the increase in the overall Salary Cap.
While still commonly referred to as the Rookie Cap, the new “Year One Rookie Allocation” has commonly been referred to as a “Cap within a Cap” because while it operates as a cap on what teams can spend on their rookies, it does not have a dollar for dollar impact on the team’s overall Salary Cap. Because of this, when you hear that a team has a $4.5M “Rookie Cap”, that doesn’t mean that the team needs $4.5M in overall Salary Cap space. As this article (link to article) explains, the overall impact is far less than the amount of the team’s Rookie Cap.