Properties of Link entity

[Mo0812]

Hey there,
I currently work on implementing the HAL specification along side with an API I build for a customer. Digging through the documentation and specification I found the description of the Link enity as following:

Links
Links have:

A target (a URI)
A relation aka. 'rel' (the name of the link)
A few other optional properties to help with deprecation, content negotiation, etc.

What other optional properties are ment here? Could it be any type of additional property or are they defined somewhere else?

Best Regards!