Q:

What is SYS token?

I've noticed it on testnet. 
Is this symbol reserved only for testnet or maybe this is an internal symbol for native token on every eosio based blockchain?

A:

It will be the native token symbol. It has been renamed from EOS to SYS.
This is to prevent confusion (even though it might add to it). The intention is that if you start a test net or fork the project, you will not use the EOS token name by default just by running the code.
In the Dawn 4.2 version, it states that it can be configured prior to running the code.

Renaming of the Core Token
Please be aware that we have changed the name of the token symbol from “EOS” to “SYS”.
If you would like to
change the name to something different, check CORE_SYMBOL_NAME in the
“CMakeLists.txt” file.

