http://wiki.soxprospects.com/Rule+5+Eligible+Players

A player is first subject to be selected in the Rule 5 Draft in the fourth or
fifth draft following the date of that player's original minor league
contract. Whether that player will be eligible in the fourth or fifth year
depends on that player's age at the time of his original contract. Players on
the 40-man roster are not eligible for the Rule 5 draft, nor are minor league
free agents. For information regarding players selected by and from Boston in
the draft, see the SoxProspects Rule 5 Draft History Page.