Under GST software goods and services are considered as inter state trade and zero rated. Hence, there is no tax on software related exports.
Check out:
http://www.charteredclub.com/all-about-gst-on-export-of-software-service/
GST UIN means GST iUnique ID. It is different from GSTIN.
GST UIN can be granted to Consulates, Embassies and other specialised agencies as mentioned above.
ITC means Input Tax Credit.
Input Tax Credit (ITC) under GST is eligible under the following conditions:
https://cleartax.in/s/input-tax-credit-under-gst