[Pharo-dev] What, exactly, does `#=` mean?

Anyone know of a book that goes in depth explaining \`#=\`? I've always wondered what "equal" is supposed to mean. For example, if I have two objects representing the same person, but one has their webpage cached as state, they represent the same entity, and are interchangeable in many contexts, but also have a difference that may be important in other contexts. Is “equal” whatever you say it is in your domain, or is it a Smalltalk object system concept? If the latter, what is the exact expectation?