Big difference. Ladkah, Sikkim And Arunachal are Indian provinces, while Tibet is not. And of course, nothing called South Tibet exists in this World.
Then you should no better than talk about it.
When was the last time someone tried to impose that upon you ?
Just to give you a very brief idea of what caste system is, it evolved as a methodology of identifying different groups of people based on their profession. So, those whom we call scholars today, were known as Brahmin, those who were the soldiers were called Kshatriya and so on. I will say, it was a brilliant idea that was formulated thousands of years back, to account for each and every person and their duty towards the society. And that system exists in each and every civilised society in this World today. The system had been subject to abuse, yes, and the way it is, social strata has been abused by each and every race, culture and civilisation, no single exception to this.
You get to talk about it, because we have preserved our way of life and documented each and every part of it, unbiased.
If you really want to talk about a subject, then you should know a thing or two it about it first.