Company details is where you manage your companies details.  The information recorded here appears in every other part of Tally.  Access this screen by going to Settings > Company Details.  The address details will appear on your invoices and other documents, as will your phone numbers.  Your e-mail address and contact name will be used for e-mailed items and for communications from Tally, this will also be where we send any password resets.  The VAT number will be used when filing VAT electronically, please ensure this is exactly as HMRC has issued you it.  The localisation options allow you to set a different date format, change the home currency and symbol and position it to the right of an amount.