In the NFL responsibilities can vary wildly within a position.
Take wide receiver. Players are given the same label even though their roles vary wildly. One could argue there are at least three different wide receiver positions.
On each play the offense must have seven players on the line of scrimmage. Five of these players are on the offensive line.
At least one of the other two is typically a wide receiver. This player in called the X receiver or split end.
The league is changing, but these are players who you would think of as traditional number one receivers. These guys tend to be big and strong. Since they are on the line of scrimmage, they have no cushion against aggressive press coverage. They have to be able to beat a jam. (For the sake of simplicity I’m going to use the same picture here. Just imagine the corner is playing tight up against the line or use this illustration.)
Typically these guys will have the speed to get deep. The outside part of the field is typically where there is no safety help for corners so a fast receiver who gains separation creates a big play on vertical routes.
The slot receiver is typically lined up behind the line of scrimmage and inside another receiver.
Jamison Crowder plays this role for the Jets and is a good template. Crowder’s lack of size doesn’t prevent him from playing in the slot. Because he typically starts behind the line of scrimmage, he is difficult to jam.
For a receiver lined up in the slot short area quickness is more important than deep speed. Unlike outside receivers, slot receivers don’t have the sideline at one side. They can break either way. Instead of having 20-30 yards to beat a defender vertically, their responsibilities tend to be shorter routes cutting either left or right. The ability to create separation with one quick burst is often the difference between getting a reception and not.
Like any receiver, slot guys will be asked to go deep at times. This is another area where Crowder makes a good template. He doesn’t really have great speed relative to other wide receivers. He would struggle to gain separation running down the field against many cornerbacks, but his deep path from the slot takes him down the middle of the field. That is the area linebackers populate, and Crowder is faster than them.
Finally we have the Z receiver or the flanker. Going back to what we said above, the offense must have seven players on the line of scrimmage. Five are offensive linemen. The X receiver makes six. On many plays the tight end is the seventh, meaning another outside receiver can line up behind the line of scrimmage.
Like the X receiver, the alignment on the outside means there is the opportunity for speedy receivers to run deep routes.
Unlike the X receiver, since these guys are behind the line of scrimmage they have more of a natural cushion from corners and are more difficult to jam.
Smaller deep threats like DeSean Jackson and Brandin Cooks have done a lot of their damage from the Z spot in their careers. This is why it is not necessarily accurate to typecast any undersized receiver as confined to the slot. These players might be out as an option for the X spot, but if you have good deep speed and/or route running ability, the Z position can be a logical fit.
Since Z receivers are off the line of scrimmage, they can also be put into motion, which means more inside breaking routes.
They might even go all the way into the slot meaning slot receiver skills are also relevant for a Z receiver.
What does this all mean?
If we were talking about the NFL of yesterday I could probably end the article here, but everything above is an extreme oversimplification of how things work in today’s league.
There aren’t hard and fast rules about the skills needed to thrive. There are always exceptions. Odell Beckham, Jr. isn’t huge, but he has thrived as an X receiver. His athleticism makes it difficult for corners to stick a jam, and his playmaking ability punishes those who miss those jams.
In recent years we have seen the rise of bigger players in the slot. Teams frequently use skilled receiving tight ends there to take advantage of their size and give their quarterbacks easy targets. In Arizona under Bruce Arians Larry Fitzgerald moved to the slot in part because his bigger body could aid blocking in the run game.
In the past you could have said a certain player’s role on many teams was X, Z, or slot. Now your role might be X, Z, or slot for a play, and then you move to a different role for the next play. NFL offenses are becoming more versatile and more diverse.
An offensive coordinator might want to move his best receiver from the X to the slot just to get him a more favorable matchup for a few plays.
Formations allow different looks as well. Adam Gase’s offense with the Jets frequently has two outside wide receivers on the line of scrimmage and the tight end behind the line. Sometimes Crowder is lined up inside but on the line of scrimmage while the guy outside is behind the line.
Much of this is philosophical. Some coaches adhere to the X, Z, and slot definitions more than others. Rams coach Sean McVay has spoken about how he likes to keep things simple for his players and generally likes to keep receivers in the same role.
As offenses continue to evolve, though, you will likely see fewer and fewer receivers only play one of these spots. I think the evaluation process for most teams will no longer ask whether a player is an X, a Z, or a slot. Instead the question will be whether he lacks the skillset to perform in any of the three roles. It will likely be a mark against him if so because it will take away from the offense’s versatility.