Imagine you run a handicraft business in Jaipur and you get a stall at a trade fair in Mumbai for 3 weeks. You're not permanently based in Maharashtra, so you can't use your Rajasthan GSTIN there. You need a temporary registration — and that's exactly what Section 27 is about.
Section 27 covers two special categories: a Casual Taxable Person (CTP) — someone who occasionally supplies goods/services in a state where they have no fixed place of business — and a Non-Resident Taxable Person (NRTP) — a foreign person or entity supplying in India without a fixed establishment here. Both are treated similarly under this section.
Here's how the registration works: The certificate is valid for whichever is shorter — the period you mention in your application, or 90 days from the effective date of registration. So if you apply for 45 days, your registration lasts 45 days. If you apply for 120 days, it still only lasts 90 days. However, the proper officer can extend this by another 90 days on sufficient cause — meaning the absolute maximum validity is 90 + 90 = 180 days. Crucially, you cannot make any taxable supply until the certificate is actually issued — no selling before registration comes through.
The big catch — and this is exam gold — is the mandatory advance deposit of tax at the time of application itself (not later). You must estimate your GST liability for the registration period and deposit that full amount upfront. If you later seek an extension, you deposit an additional advance for the extended period. This amount gets credited to your Electronic Cash Ledger and is utilised as per Section 49 (normal payment rules apply). Think of it as a security deposit the government holds — it protects revenue since CTPs and NRTPs are transient and might disappear after the event. This is asked frequently as a 4-mark question in CA Inter exams, especially the advance deposit angle and the 90+90 day rule.