Section 13 stipulates that following goods are exempt from levy of sales tax:
⇒ Supply or import of goods listed in sixth schedule
⇒ Goods specified by Federal Government through its SROs to the extents and
from the date specified therein
The sixth schedule includes a list of items on which no sales tax is levied.

Following is the excerpts  of section 13 of Sales Tax Act 1990;

13. Exemption.–

(1) Notwithstanding the provisions of section 3, supply of goods or import of goods specified in the Sixth Schedule shall, subject to such conditions as may be specified by the Federal Government, be exempt from tax under this Act .

(2) Notwithstanding the provisions of sub-section (1) –
(a) the Federal Government may, pursuant to the approval of the Economic Coordination Committee of Cabinet, whenever circumstances exist to take immediate action for the purposes of national security, natural disaster, national food security in emergency situations, protection of national economic interests in situations arising out of abnormal fluctuation in international commodity prices, removal of anomalies in taxes, development of backward areas and implementation of bilateral and multilateral agreements by notification in the official Gazette, exempt any taxable supplies made or import or supply of any goods or class of goods, from the whole or any part of the tax chargeable under this Act, subject to the conditions and limitations specified therein;

(3) The exemption from tax chargeable under sub-section (2) may be allowed from any previous date specified in the notification issued under clause (a).

(4) Sub-section (4) omitted by Finance Act, 2006.

(5) Sub-section (5) omitted by Sales Tax (Amendment) Ordinance, 2000.

(6) The Federal Government shall place before the National Assembly all notifications issued under this section in a financial year.

(7) Any notification issued under sub-section (2), after 1st July, 2015 shall, if not earlier rescinded, stand rescinded on the expiry of the financial year in which it was issued.