Thank you mashahood_niz. Most domains can only be registered for 10 years at a time. However, if you keep renewing your domains, you can keep your domains for more than 10 years.
What happened when i own a domain for more than 10 years? Will it automatically dropped and can be registered by someone?
You may have not fully understood Support's reply, they mean that a domain (.com, .net, .org etc Full list here) cannot be registered for more than 10 years at a time. You can own a domain for what ever period you want.
Assume I register a domain "mydomain.com" at some other registrar today for 10 years, It mean it's expiry date will be on Feb 6, 2018. After two months I decide to move my domain here, I will have to pay 8.99 for the transfer but the expiry date will remains the same (Feb 6, 2018).
Normally if you transfer a domain you will get an extra year of renewal.
Most domains can only be registered for a maximum of 10 years. If you transfer a domain that has over 9 years of registration on it (9 years and 1 month, for example), the 1-year renewal included with the domain transfer will not be added to your domain.