What are the consequences on conclusion of provisional assessment by way of passing final assessment order in so far as short / excess remittance of tax is concerned

The consequences on concluding the provisional assessment by way of passing final assessment order would be as follows:
(a) Additional tax liability: In case of short remittance of taxes in terms of final
assessment order, the additional tax liability, if any should be remitted along with interest at the rate prescribed under Section 50(1) for delay in remittance of taxes viz., from the first day after the due date of remittance of taxes as prescribed under Section 39(7) till the date of actual payment;
(b) Excess remittance of tax on provisional basis: In case of excess remittance of taxes in terms of final assessment order, the registered person is entitled to refund of such excess remittance in the manner as provided in Section 54(8) along with interest as provided under Section 56.

(ICAI FAQ PUBLICATIONS 06-09-2017 PROVISIONAL ASSESSMENT: FAQ NO. 5)

What is the time limit for passing final assessment order in case of provisional assessment

The proper officer shall, within a period of six months from the date of communication of the provisional assessment order, pass the final assessment order after taking into account such information as may be required for finalizing the assessment. However, the time limit of six months can be further extended on sufficient cause being shown and for reasons to be recorded in writing in the following manner:
(a) by the Joint / Additional Commissioner for a further period of six months;
(b) by the Commissioner for such further period not exceeding four years.

(ICAI FAQ PUBLICATIONS 06-09-2017 PROVISIONAL ASSESSMENT: FAQ NO. 4)

How is the assessment made if the taxable person is not able to determine the value of goods and/or services or determine the rate of tax

Where the taxable person is unable to determine the value of goods or services or both or determine the rate of tax applicable thereto, he may request the proper officer in writing giving reasons for payment of tax on a provisional basis and the proper officer shall pass an order, within a period not later than ninety days from the date of receipt of such request, allowing payment of tax on provisional basis at such rate or on such value as may be specified by him.

(ICAI FAQ PUBLICATIONS 06-09-2017 PROVISIONAL ASSESSMENT: FAQ NO. 1)