You can set up that Your account can be used on any chain. This way You will not need to worry about this option when You switch between various networks. That’s why I recommend using this option. In addition, new networks in the Polkadot ecosystem are not usually visible in this box of polkadot.js extension. At this stage You can choose the main live networks like Polkadot, Kusama, HydraDX or Plasm (currently Astar) or simply leave it as it is. Finally You can click the button indicated by the arrow and Add the account with the generated seed.