Steps:

1. Start a server and join it as op in creative
2. Type these commands: (So your scoreboard is prepared)

   /scoreboard objectives add Test dummy
   /scoreboard objectives setdisplay sidebar Test 0

3. Type these commands: (You are the only tracked player)

   /scoreboard players set <yourPlayerName> Test 0
   /scoreboard players set <friendPlayerName> Test 0
   /scoreboard players set <non-existantPlayerName> Test 0

4. Type these commands: (This only change your score)

   /scoreboard players set @a Test 1

5. Let a friend join the server
6. Type these commands: (Your friend is tracked)

   /scoreboard players set <friendPlayerName> Test 1

7. Type these commands: (Your friend score and yours is set to 2)

   /scoreboard players set @a Test 2

8. Tell your friend to leave the server
9. Type these commands: (You can't change his score when he's offline, whenever he's tracked)

   /scoreboard players set <friendPlayerName> Test 1

10. Type these commands: (You can only change the score of online players, whenever other are tracked)

    /scoreboard players set @a Test 0

It's only possible to change score for online tracked players.

• Having the possibility to change the score an already tracked players.
• Having @a selectors to change the score of all tracked players of the specified objective, additionaly to connected players
• Having the possibility to add an offline player to the list of tracked player for one objective, if their names is given explicitly.

• Since a player is tracked for a scoreboard and he's not on the server,
• It's not possible to change his score, when we use his name explicitly or with selectors such as @a.
• It's not possible to set a name for a player that's not on the server or non-existant player.