SN & LH use Amadeus Altea DCS' theoretical seating, so you already have a seat assigned to you (together with anyone in your party). Once the checkin opens (officially at T-24h, sometimes already the day before departure) that seat is shown to you in the online checkin (web, app etc) process. If you have others travelling with you on the same intenary but on different reservations, you can add them to your checkin process and pick seats together. During that process the system lumps you and the other party together and lets you pick seats together. This link however is lifted So if there is a change in aircraft etc the seating will be allocated again, but not together with the party in the other booking.