Below the explanation of the account settings page
Public picture will be shown at the top banner next to your login details
Company logo will be shown when sending invoices and quotes via the CRM system
Your company and bank details are printed on invoices.
VAT: This is your local sales tax which will be shown on invoices. In case you are not adding VAT (i.e. cross border sales) you can set it to 00.00
You need to enter your company VAT number, this is mandatory.