This is something that people have been asking me offline and via email "What is a Rotational player?" It made so much sense to me that I have answered it on a one off basis, but the question has come up often enough that I feel it is time to answer it here.
As a little bonus I offer up for your discussion my first attempt at a Depth Chart, though not graphically. These are my opinions, but would love your input. First though we will look at my categorization of the players on a squad.
Starters - These are the players that if the entire team is healthy and allowed to play would be starting. Probably the most obvious of my terms relating to the depth of the squad.
Rotationals - These are players that it seems to me that Sigi wants to use as either a starter or regular substitute. They get in nearly every match when healthy and are capable of starting not just in a pinch by are solid additions to the club. The strongest example of this for the Sounders is Sebastien Le Toux, but also Patrick Ianni and Nathan Sturgis (at least early in the season). For the Sounders these would be players used in the Open Cup proper. Ideally this set would be 3-4 deep.
Substitutes - Obviously these are guys regularly on the available bench and getting used. That's the key. Playing time. These are the players that a team can count on in a pinch. You wouldn't want them to start two weeks in a row, or to use them four weeks in a row for a lot of minutes. Sounders in this role are Sanna Nyassi, Stephen King, and Roger Levesque. They are key elements of depth and for Sounders were used in Open Cup Qualifiers. This tends to be the next 3-4 strongest players.
Reserves - The second most obvious set of players. This are players that only get time in MLS matches when the team is hit with significant injuries, suspensions or call-ups. This set of players will tend to only get time in Reserve matches and Friendlies. For the Rave Green these have been Mike Fucito, Evan Brown, Terry Boss, Chris Eylander, Lamar Neagle and formerly Jarrod Smith. The final 6-10 on roster (depending on the DL).
A "first team" would be the Starters+Rotationals and a "second team" would be Subs and Reserves.
The Depth Chart (as I see it)
First do you remember Sigi's Arrow? This graphic should help clarify the roles but please remember the Interchange that has become so popular lately for Sigi. The starting roles are only guides.
Target Forward - Nate Jaqua, Seb Le Toux, Roger Levesque, Mike Fucito
Withdrawn Forward - Fredy Montero, Freddie Ljungberg, Steve Zakuani, Sanna Nyassi
Left Wing - Steve Zakuani, Freddie Ljungberg, Seb Le Toux, Mike Fucito
Right Wing - Freddie Ljungberg, Seb Le Toux, Fredy Montero, Sanna Nyassi
Central Midfielder - Brad Evans, Osvaldo Alonso, Stephen King, Peter Vagenas
Defensive Midfielder - Osvaldo Alsonso, Patrick Ianni, Peter Vagenas, Nathan Sturgis
Left Wingback - Leo Gonzalez, Zach Scott, Tyson Wahl, Nathan Sturgis
Right Wingback - James Riley, Patrick Ianni, Brad Evans, Zach Scott
Centerbacks - Jhon Hurtado, Tyrone Marshall, Patrick Ianni, Taylor Graham, Tyson Wahl, Zach Scott, Nathan Sturgis
Keeper - Kasey Keller, Chris Eylander, Terry Boss
At this point if a player isn't one of the four best/most likely to be used at any position they likely are going to seriously struggle for time.