igraph对象的可变部分是什么？

Question

igraph文档提到了igraph对象的“可变”部分。

我无法找到关于那是什么的讨论。

来自documentation

identical_graphs:
 Decide if two graphs are identical
Description:
This is similar to identical in the base package, but ignores the mutable 
piece of igraph objects,
that might be different, even if the two graphs are identical.

这是显而易见的且易于解释的吗，还是在此背景下有很好的资源？