In conversations I often hear people using the word “mutable”. Typically it would be something similar to the following.
The IP TTL is a mutable field and decreases as it traverses a router.
If we look up the word “mutable” we find that it is something that is likely to change.
So the IP TTL is a great example of a mutable field. That value is expected to decrease at each hop throughout the network.
By contrast, IP Source Address, Destination Address, and Protocol would be immutable because it is not normal for those to change. They should stay constant from source to destination.
I know someone might consider something like NAT or PAT to change some of these attributes. While that is true, I still would not consider those mutable fields.