You can choose different EOA account addresses to initiate transactions with. Under the Account drop-down list, it shows you the EOA accounts connected with Remix. As seen, the Remix instance is connected using MetaMask and an EOA that starts with 0x339; this also has a balance of 1 ether on the Ropsten test network. You can choose between multiple accounts that you want to use depending on your requirements.

If you are using the Injected Web3 environment and are connected with MetaMask, this drop-down list will only show one account in the list–that is, the account you specify from MetaMask itself. Once you change the address on MetaMask, it will automatically be reflected in the Remix IDE and the account will be changed. However, if you are using the JavaScript VM environment, it would populate the drop-down list with some random account addresses; each will have 100 ether balance and you can choose different accounts from this list.