This is word for word from the user manual.

"The other limitation is on playing players out of their natural position. When you select a position for your depth chart, you will be given a popup list of every player who is eligible for that position. It's generally limited to players who could possibly outperform a street free agent at the position. However, that's not a guarantee. The bottom line is that you can't put every player on your roster in every position. It has to be a situation that could possibly make sense. There will be no quarterbacks at nose tackle, for example, even if it's that 300-pound quarterback that the Giants had a few years back.

When you select a spot on your depth chart and the popup list appears, you'll probably notice that some names are "grayed out" and not possible to select. This is because they are already selected on the depth chart at another position. If you really need Sammy Baugh at quarterback and he's grayed out, take him off the depth chart at punter or defensive back. We don't allow two-way players in this league.

So what if you don't have enough players to fill the depth chart? Either you don't have that third tight end on the roster, or you have injuries, or you just don't have the right talent set? There's a simple answer for that, and you don't have to worry about having only 10 men on the field. Holes are automatically filled by "street free agents".

The software will automatically identify your top kicker, punter, kick returner, and punt returner, so you don't need to worry about those positions."

Seems like you can put a player anywhere you want but they won't play as well out of position. Cant have a player backing up a position that is already starting in another position, and can't have a player playing both sides of the field.

Most questions that I see on here are usually answerable on the users manual page. Id suggest starting there next time you have Q.