On the tactics page you will find 7 spaces for Substitutes. When a player is located in the substitutes he can be used within your adjustments.
If a player is not listed as a Substitute he cannot be used in your adjustments. Furthermore if the player is no longer a Substitute, but still listed in the adjustments, the adjustment will fail. You can read more about adjustments above.
If a player has to be substituted and there is no adjustment made for that player, the system will calculate the best suited player for this position. Every player on the substitute list, that has not yet been on the field, will get a value calculated, and based on this the system will select the best player.