As to the designations of receivers one, two and three: Jackson, even as the "fourth" receiver off the bench, has an opportunity to catch forty plus balls, regardless of whether or not he platoons with Gaffney. He will have the opportunity in training camp, and throughout the season, to win the third starting receiver role. If he progresses as I think he can with a full offseason working out with the team, coupled with the wealth of knowledge at his fingertips with three of the top offensive players in the league around him,* expect Jackson to be a starter when week 1 comes around.
I agree with you, but I find the use of numbers for receiver positions to be annoying (generally, not directed at you). People use the numbers in different contexts to support whatever they are talking about.
Do the numbers reflect the number of catches? Then Moss is a #2 receiver and KC/Philly don't have a #1 "receiver".
Do the numbers reflect a position on the field like X, Y and Z? That makes Welker a #3 receiver.
Do the numbers reflect skills, contract terms or some other subjective evaluation? TJ and CJ in Cincy would love that.
The numbers are generally worthless anyway but when discussing WR roles they just get plain confusing. What is the difference between a #2, #3 and #4 when discussing CJack? The amount he gets on the field? That is subject to the gameplan that day. The number of catches he gets? He sure won't be a #2 in that case, but catches depends on many factors not known ahead of time. Whether he starts or not? Depends on the formation to start the game.
Welker is a slot/Y receiver. Gaffney is really not well suited outside so he likely falls here too for his primary responsibility...as well as opposite Welker in the 4-wide set.
Moss is a split end/X receiver. Gets him away from the clutter and forces a defense to shade his side of the field or else leave the CB in a physical mismatch on an island. Washington likely fits in this role as well.
CJack is a flanker/Z receiver. Set off the line, sometimes in motion to attack multiple areas of the field quickly.
There are just rough designations, not hard-n-fast rules. I would like to see Moss off the line and in motion more this year. If CJack doesn't step up, you will likely see Gaffney as a flanker, just like he did replacing Stallworth late in 2007.
The point is...CJack is trying to claim the flanker role and it is his for the taking. Nobody knows if he will take the next step and claim the job. Whatever the case, assigning numbers to WR roles should probably be accompanied by an explanation of the point you are trying to make. Saying CJack is a #2 receiver may or may not make sense depending on your definition.