When you have multiple Core companies, you might prefer Core to open a specific company when logging in. You can do this by opening the company menu > Account > Manage Companies. On the Manage Companies screen, select a company on the grid and enable the 'Default' option for that and Core will automatically open this company when logging in. You can switch between companies from the company menu by clicking 'Switch Company' and selecting another company from the list. Check out Core Help for details.