Sync on load question

Hello there, im trying to make a forge 1.20.1 server and well, I m trying to squeeze every inch of optimization I can tbh. Checking Distant Horizons settings I stumble upon this sync on load mode. Although I may ask, when a players logs into the server, the server sends all LODs (based on settings), and later update the ones that had been changed during the player's connection? Or when the player logs, it loads a local version of the LODs of the server (like Bobby lets say), and then updates the LOD via sync on load? I'm just curious if the later option/functionality exist so server side load can be lighter