Polkadot.js allows you to interact with different blockchains (Polkadot, Kusama and several others). It is theoretically possible to use one account on different chains, however we do not recommend it as it may lead to confusion.

We therefore recommend that you restrict your account to only one network. That's why the polkadot.js interface gives you an alert for any account that you have not done this for yet. 

To make sure you're only using your account on the current network, click on the three dots next to your account on the accounts page, and then select "only this network" from the menu:

If you are managing your account with the browser extension, you can select the right chain in there by clicking on the settings wheel and then "Display address format for"

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