Is there any way to check the code is Short Code or long Code in the Mobile Connect?
what are the merits and demerits of both code?
I found a few articles but they where confused me more.
Short Code is - well, short (e.g. 1234) and is very often shared across multiple accounts on SFMC, as dedicated short codes are - however possible, really costly.
Long code is a standard local phone number (e.g. +4511223344) and are normally dedicated to a single account.
It is indeed better to use a short code for inbound messaging, as they are easier to remember. However your option of media codes will also be limited, as you need to choose something unique across the entire short code. Also, the default STOP keyword (I.e. texting STOP to 1234) will opt out of ALL communication across ALL accounts using same short code.
Hence I will recommend a dedicated long code, unless you rely heavily on collecting inbound texts.
Long codes are basically meant for one to one conversations (they are limited to 1 second per message by SFMC). They are necessary for international messaging as well.
You should use a short code for high volume sends (up to 100 messages per second).