1. No player may be optioned to a minor league club whose regular season (including post-season playoffs) has concluded.
2. No player may be optioned to the minors prior to the start of Spring Training.
3. An injured player cannot be optioned to the minors.
4. A player who is optioned to the minors generally must remain on Optional Assignment for at least ten days. (A player optioned to the minors during Spring Training generally must remain on Optional Assignment for the first ten days of the MLB regular season). The only exceptions are if the player is traded or claimed off waivers, recalled as the "26th man" for a doubleheader (second game of the doubleheader only if two games were scheduled after a postponed or suspended game from the previous day), or is recalled to replace a player on the MLB Active List (25-man roster) who is traded, claimed off waivers, or placed on the MLB Injured List, Bereavement/Family Medical Emergency List, Paternity Leave List, or Restricted List (but only pursuant to a suspension or administrative leave resulting from a violation or possible violation of the joint MLB-MLBPA JDV policy or a suspension resulting from a violation of the joint MLB-MLBPA JDPTP policy).
NOTE: If a player is acquired in a trade and is optioned to the minors by his new club within 24 hours, the player can be recalled from the Optional Assignment at any time without restriction, but if a player on Optional Assignment to the minors is claimed off waivers and is then optioned to the minors by his new club, the ten-day limit does apply. So as far as the ten-day limit is concerned, there is a distinction made between acquiring a player in a trade and claiming a player off waivers.
5. All players on Optional Assignment to the minors must be recalled no later than the day after the conclusion of the MLB regular season.
6. A Rule 6 "Selected Player" cannot be optioned to the minors until restrictions have been removed.
RULE 6 SELECTED PLAYERS WITH MINOR LEAGUE OPTIONS REMAINING (updated 3-28-2017):
NONE AT THIS TIME
7. Beginning on the day after the conclusion of the MLB regular season, a Rule 6 "Draft-Excluded Player" cannot be optioned to the minors any earlier than 20 days prior to MLB Opening Day.
RULE 6 DRAFT-EXCLUDED PLAYERS WITH MINOR LEAGUE OPTIONS REMAINING (updated 3-8-2019)
NONE AT THIS TIME
8. Per Article XIX-A of the CBA, any player on an MLB 40-man roster who has accrued at least five years of MLB Service Time (as well as an international player with Article XIX-A contractual rights) has the right to refuse an Optional Assignment to the minors, or the player can elect to be a free-agent immediately upon being optioned (he has two days to decide).
If a player eligible to elect free-agency is optioned and elects to be a free-agent immediately, his contract is terminated and he receives no termination pay.
If a player with Article XIX-A rights refuses an Optional Assignment but does not elect free-agency, the club must either retain the player on the club's MLB Reserve List (40-man roster) AND Active List (25-man roster), trade the player, or give the player his Outright Release.
A player with Article XIX-A rights can waive his right to refuse an Optional Assignment in advance, but the waiver cannot be signed more than ten days prior to MLB Opening Day, the player has the right to designate in advance which minor league team to which he can be assigned, and the waiver automatically expires if the player is not optioned to the minors within 45 days or by the 45th day of the MLB regular season (whichever is later).
CUBS ARTICLE XIX-A PLAYERS WITH MINOR LEAGUE OPTIONS REMAINING (last updated 7-31-2019):
Tony Barnette, RHP (vested foreign player)
Nicholas Castellanos, OF
Steve Cishek, RHP
Yu Darvish, RHP
Daniel Descalso, INF
Cole Hamels, LHP
Kyle Hendricks, RHP
Jason Heyward, OF
Derek Holland, LHP
Craig Kimbrel, RHP
Brandon Kintzler, RHP
Jon Lester, LHP
Jonathan Lucroy, C
Brandon Morrow, RHP
David Phelps, RHP
Jose Quintana, LHP
Anthony Rizzo, 1B
Ben Zobrist, IF-OF