12/2 UPDATE #2: Mark Gonzales at the Chicago Tribune is reporting that the Cubs have signed arbitration-eligible LHP Clayton Richard to a 2016 contract ($2M salary), and have non-tendered RHRP Ryan Cook and LHRP Jack Leathersich. The Cubs claimed both Cook and Leathersich off waivers last month, Cook from the Boston Red Sox and Leathersich from the New York Mets.
The other twenty unsigned players on the Cubs MLB 40-man roster were apparently tendered 2016 contracts, and six of the twenty (RHSP Jake Arrieta, OF-IF Chris Coghlan, RHRP Justin Grimm, RHRP Hector Rondon, RHRP Pedro Strop, and LHP Travis Wood) will be eligible for salary arbitration if the player is not satisfied with the club's salary offer.
Cook was eligible for salary arbitration for the second time and was projected to get about $1.5M for 2016 (he was paid $1.4M in 2015), and the Cubs may not have wanted to pay him that much if he is expected to spend most of the year as "bullpen injury insurance" at AAA Iowa. So the Cubs could possibly re-sign Cook to a major league contract with a minimal base salary (perhaps $750K) plus a performance bonus based on days spent on the MLB 25-man roster in 2016 (something they could not do if he was tendered), or they could sign him to a 2016 minor league contract with an NRI to Spring Training. Whether Cook would agree to such an arrangement remains to be seen, but the Cubs might not have non-tendered Cook if they didn't already have a 2016 deal in place.
Leathetsich is not yet eligible for salary arbitration, but he could not be outrighted to the minors during the off-season (after 11/20) because he is injured (July TJS), so non-tendering him and then re-signing him to a 2016 minor league contract is the only way the Cubs can remove Leathersich from their MLB 40-man roster but still keep him under club control. Of course the Cubs could have simply waited until the start of Spring Training and then placed Leathersich on the 60-day DL (thus removing him from the 40-man roster), so it would appear that the Cubs believe they will need his roster slot on the 40 before then. It remains to be seen if Leathersich will sign a minor league contract with the Cubs, but (as with Cook) the Cubs probably would not have non-tendered him if they did not already have a pre-arranged minor league deal in place.
So the Cubs MLB Reserve List (AKA "40-man roster") now stands at 36, with four slots left open for free-agents who might be signed over the next few days and weeks, off-season waiver claims, or perhaps even a Rule 5 Draft pick.
12/2 UPDATE #1: Jon Heyman at CBS Sports reports that the Cubs have signed LHRP Rex Brothers to a 2016 contract ($1.42M salary). Brothers was paid $1.4M in 2015, and would have been arbitration-eligible for the second time post-2015. He was acquired by the Cubs from the Colorado Rockies for minor league LHSP Wander Cabrera last month.
Tomorrow (Wednesday December 2nd) is the deadline for MLB clubs to tender 2016 major league contracts to unsigned players on the club's MLB Reserve List (AKA "40-man roster").