Sleek Bill Online has full support if you deal with clients from special economic zones or union territories in India.

Specify if a client is from a SEZ zone

When adding a new client from https://app.sleekbill.in/clients/new, tick the box that says "Special Economic Zone (SEZ)" as shown below:

The current special econimic zones in India can be found here: https://en.wikipedia.org/wiki/List_of_special_economic_zones_in_India

Union territories support

If your company is from any of the current union territories in India:

In Sleek Bill Online, when you create invoices or other documents, the SGST part of the GST rate will automatically appear on your invoice as UTGST.