All players are technically visible to all users. A world could have over 1000 eligible Rule 5 players and are ranked according to your Rule 5 draft settings. The lower the number for the weight for a position, the more likely they will be low on the draft board.
When the board is initially created, and when target, weight and types of the positions wanted on the settings page are changed, all players are re-ranked.
At maximum, 500 players are shown even though a maximum of 320 players can be drafted (32 franchises with a maximum of 10 rounds). If the player falls below 500, you will no longer be able to see them (unless you change your settings).
For example, if you gave SS’s a weight of 0, gave every other position a weight of 10, non-SS's would be at the top of the list and SS’s would be at the bottom. If there are 500 or more non-SS's eligible for the Rule 5 draft, SS’s won’t be displayed.