Csdrhrt

The definition of ICMP message says that it has a type, a code and the first 8 bytes of IP datagram causing error.

Question: Which first 8 bytes of IP datagram are included in the ICMP datagram?

My understanding: The portion marked by the red box in the figure below.

Well, the sender might like to know which ones of its packets the ICMP message refers to. For that, the entire IP header plus (at least) the first eight bytes of the IP payload are appended to the ICMP header for error messages.

I am not sure what definition you are referring to (source?), but it probably meant a 8 bit.

An ICMP packet is 8 bytes total[1], the first 16 bits are type and code bytes(eight bits in a byte); the type byte is given a number from 0 to 255[2], a different number representing a different type of sub-code.

As Zac67 stated the sender usually likes to know where the ICMP packet comes from, so it encapsulates it in an IP header, that IP header contains a field called 'protocol' when an ICMP packet is carried in the payload the protocol field is set[3] to 1 (0x01 in hex or 00000001 in binary), the protocol field is also bits (1 byte) long.

Below is some useful resources and sources.
Hope this helps.

https://en.wikipedia.org/wiki/Internet_Control_Message_Protocol

http://slideplayer.com/slide/3602717/

http://www.enterprisenetworkingplanet.com/netsp/article.php/3584166/Networking-101-Understanding-and-Using-ICMP.htm

http://www.pearsonitcertification.com/articles/article.aspx?p=1804873

[1] - http://www.keyboardbanger.com/wp-content/uploads/2015/07/ICMP-header.png

[2] - https://en.wikipedia.org/wiki/Internet_Control_Message_Protocol#Control_messages

[3] - https://en.wikipedia.org/wiki/List_of_IP_protocol_numbers