The Argentine style of asking for a dance is that the leaders ask. They ask in a particular way, often from across the room, by seeking eye contact with followers. If eye contact is held, the leader nods toward the dance floor (or raises one or both eyebrows). This is the cabeceo.
There is no requirement to change partners, but it is recommended that you dance with lots of different people, even if you have a main partner.
The codigos guide social interaction when tango is being danced. They respect the sanctity of the dance and take care of dancers’ feelings and egos.