Analytic account field on timesheet is your Contract/Project on which you are filling the timesheets. So you are encoding timesheet on that analytic account and later you can invoice to your client.

Analytic Account mainly used for costing purpose in OpenERP. For example if the IT company has create analytic account for some projects (R&D or Offshore) and the user is working on R&D department then he has to fill the timesheet in that analytic account. So in timesheet user has to record the works he/she has done for that Analytic Account/ Project. Same time you can create analytic account on which you can raise invoice to your client for example offshore project so whatever timesheet done on that offshore project/ Offshore analytic account company (Account Manager) can raise invoice.

Thanks, Priyesh Solanki