indicates link directs to Baseball Reference
Pos. - Position (primary position first, plus any position played for 5+ games in 2019 or 2020)
Level - Highest level reached in 2019 or 2020. Post-season-only or brief call-ups from one minor league level to another may be excepted. Player on an inactive list for entire previous season will be listed on respective inactive list even if higher level previously reached.
   A+ denotes High-A (California, Carolina, and Florida St. leagues)
   A denotes Low-A (Midwest and South Atlantic leagues)
   A- denotes Short-Season-A (NY-Penn and Northwest leagues
   YTD denotes player has yet to debut professionally
"40" Column ● denotes player is on Boston's 40-man Roster
"R5" Column ● denotes player is eligble for 2020 Rule 5 Draft
RL - Spent last season on Restricted List
PB - Piggyback
IL-45 - deontes 45-day Injured List
L = bats left (for hitters only); throws left (for pitchers only)
ƅ = switch hitter (for hitters only)

Free Agents
Major League Free Agents - Following the completion of the term of his contract, any player with 6+ years of major league service time who has not executed a contract for the next succeeding season shall be eligible to become a free agent. (Basic Agreement, Article XX(B)).
Article XX(D) Free Agents - Any player: (1) with 3+ years of major league service time and whose contract is assigned outright to a minor league club; or (2) whose contract is assigned outright to a minor league club for the second time or any subsequent time in his career may elect, in lieu of accepting such assignment, to become a free agent. (Basic Agreement, Article XX(D)).
Minor League Free Agents - On the fifth day following the completion of the World Series on the last year of a player's minor league contract, the player's contract shall expire and the player shall become a "minor league free agent" unless that player's club renews his contract.  (Major League Rule 55(a)).  Standard minor league player contracts are typically for seven seasons.