Knowledge question about safety channels

Dear Siemens Programmers, 

As beginner i constantly learn new items in the field of PLC programming. 
One probably basic question is still open about safety.

Why does Safety sensors and modules (Remote I/O ET200SP) use channels insteads of seperatly named digital in/outputs ports? 
Why does safety use 2 channels, do you always need to connect 2 channels?
How do you properly connect safety sensors on an ET200SP 6ES7136-6BA00-0CA0 input module?

Is there an simple basic (beginner) document description about safety?

Kind regards thus far, 

Lennard van der Kamp

Hello Lennard,

the answer is not so simple.

There is a first overview - Brochure-Safety-Integrated.

How do wire your sensor and what for sensors you can use is dependent of your application and your risk analysis. After the analysis you know which SIL (Safety Integrated Level) or which PL (Performance Level) do you need.
And then can you configure your PLC.

For more information of your F-DI-module please look at this link.

There can you see why do you need input and output for a DI-module (necessary for internal Failsafe-communication and monitoring). Also you can see which configuration is possible.

Hope this helps for the start.

regards
CaOd