Effectively, HMRC ask you to assess what tax you think you should pay. By submitting the return you are making a declaration that you have been honest and truthful and that you believe you have included all sources of income and only made claims to which you are entitled.
The fact that the return is processed and you have paid the amount requested by the due date does not mean that HMRC have agreed with your figures and/or calculations. HMRC could open an enquiry into your affairs for any reason any time within 12 months of 31 January or the submission date if later.
MR to Expand
Insight: Most taxes are self-assessed.