Each box is singles or doubles, and one season can hold both: boxes 1 to 4 singles, boxes 5 and 6 doubles, if that is how your club runs. You choose when you set the box up, and it is fixed once the box exists, because every score in it was played that way.
In a doubles box a pair is one entry. They take one row in the table, play one match against each other pair, and share the points. Type both names when you set the box up. Any of the four players can enter a score, and either member of the other pair confirms it, though not the partner of whoever typed it.
Somebody can be in a singles box and a doubles box at the same time. That is normal and nothing warns you about it. Being in two boxes of the same kind still gets a warning, because that is usually a name typed into the wrong one.
Naming a partner
Where a member puts their own name down rather than being typed in, they name the partner they want and it waits for that person to say yes. That is how a doubles event in a tournament is entered, and how somebody joins a doubles ladder. Nobody is put in a pair without agreeing to it.
Start typing and it offers names off your club’s own list, two letters in. It never shows you the club as a list to browse, and it never lets you invent somebody. A partner has to be a member already, so a pair is always two people the club has.
Until they answer, the entry sits as yours and waiting. If they say no, or never answer, nothing has happened and you can ask somebody else.
Which formats a doubles box plays
The same ones a singles box plays, in every sport. Padel is the one that opens on doubles, because that is how padel is played, and the others open on singles and take a doubles box whenever you make one.
Everything on the score form is the same too. A pair enters a score exactly the way a player does, and the tables count sets and games, or games and points, in the words that sport uses.
Doubles everywhere else
A doubles box is one of four places two people play as one, and they do not all work the same way. A doubles ladder can hold pairs or hold people whose partners change. A doubles event in a tournament is entered as a pair. A club night is doubles on court and an entry is still one person, because you get a new partner every round.