Today is Sept. 1, an important landmark in the baseball season, but also a day that tends to cause some confusion.
So as the homestretch of the regular season begins, here are some questions and answers regarding the various rules of roster expansion and how they affect clubs and players.
What changes on Sept. 1?
For the first five months of the season, each team has a 25-man active roster, which is composed of players on its 40-man roster. But on Sept. 1, active rosters expand, which allows a club to activate, suit up and deploy any player on its 40-man roster.
No team actually takes advantage of all of those spots, but almost every one -- contender or not -- adds a player or two, and often more, on Sept. 1. As Minor League affiliates finish their seasons in early September, more reinforcements typically arrive, with clubs seeking additional catchers, extra bullpen arms and bench bats, or a speedster who can change a game as a pinch-runner, such as the Royals' Terrance Gore.
Last season, for example, the eventual World Series champions expanded their active roster to 33 players on Sept. 1, with a group that included Gore, then added two more players about a week later. 

Are teams still allowed to make trades?
The answer is yes -- but with a significant catch.
The non-waiver Trade Deadline typically is on July 31, but it was on Aug. 1 this season. For any trades after that, a traded player must first be placed on waivers. If the player is claimed, his team can let him go, work out a trade with the claiming team or pull him back. If the player clears waivers -- meaning 29 other teams pass on his rights -- he then can be dealt to any team.
Those rules do not change in September, even though Aug. 31 is sometimes referred to as a "deadline" for waiver trades. But Aug. 31 is the deadline to acquire players in time for them to be eligible for their new team's postseason roster. For example, the Cubs picked up outfielder Austin Jackson from the Mariners last Aug. 31, and Jackson went on to play in five playoff games for Chicago. However, infielder Darwin Barney was not postseason-eligible for the Blue Jays, because they acquired him from the Dodgers on Sept. 13.

What does Sept. 1 mean for postseason rosters?
If a player is to be eligible for a team's postseason roster, he must be in its organization as of 11:59 p.m. ET on Aug. 31. Specifically, players must be on the 40-man roster, the 60-day disabled list or the bereavement/family medical emergency list at that time.
As mentioned, players acquired via trade in September are not eligible for their new team, and the same goes for those signed as free agents during that time. However, if a club wants to add a player who was not on the 40-man roster but was in the organization, it can petition the Commissioner's Office if that player is replacing someone who has been on the 60-day DL for at least 60 days.
One notable example of this was pitcher Brandon Finnegan, who appeared in every stage of the 2014 postseason for the Royals despite the fact that he was not added to the 40-man roster until Kansas City purchased his contract from its Double-A club on Sept. 1 of that year.

How do September callups affect rookie eligibility?
In order for a player to be eligible for a Rookie of the Year Award in 2017, he must be within certain thresholds at the start of next season. First, he must not have reached 130 at-bats or 50 innings pitched in the Majors. Second, he must not have spent 45 days on an active Major League -- except during post-Sept. 1 periods of roster expansion.
So the key thing to remember when prospects are called up this month is that while September at-bats and innings do count against rookie eligibility, days on the roster do not. In other words, if a prospect spends most of the final month riding the bench in the Majors, it's not going to prevent him from being eligible for the Rookie of the Year Award next season. For example, Yankees phenom Gary Sánchez was called up on Aug. 3, and because September days don't count for rookie eligibiliy, he will fall short of the 45-day threshold. However, he already has 99 at-bats and will almost certainly surpass 130 ABs before the end of the season, which means he will not be eligible for the American League Rookie of the Year Award in 2017 (he is, of course, eligible for it this year).
Does the "10-day rule" change in September?
Usually, if a player is optioned to a Minor League affiliate, he cannot be recalled for 10 days unless he is replacing a player on the big league roster who goes on the DL, the bereavement/family medical emergency list, the paternity leave list or the restricted list.
But in September, another exception emerges. If a player is optioned to a Minor League club whose season -- including any playoffs -- ends prior to the conclusion of that 10-day period, the player can be recalled at that point, with approval from the Commissioner's Office.