Ignoring the terms being used in question, maintaining dependency between two values/parameters is achieved through a feature called Characteristics in P360. I would suggest to check the behavior of it.
Coming to Length attribute and Unit , it seems Unit is mandatory for any value of Length. If so you dont you have the field Unit fixed? In case you need for UI/feel design like I suggested you can check the Characteristic values and see if that can help. It helps to maintain a dependency and hierarchical way of values in PIM.
Thanks for update. This is really strange that every other question having 'dependency or dynamic' words, answer for that revolve around Characteristics Data Model. You can't achieve each and everything from Characteristics.
Question which I have asked is very common and genuine requirement fulfilled by most of the tools/application available for PIM and yet I am figuring out how to achieve that in PIM.
I can't ask my client to Fix Units for attributes. Units should always be a drop down available along side fields/attribute and user should be free to choose the suitable Units for the value.
I can see 'Units' functionality in PIM, I tried that for Structure Feature and as well as for Characteristics and found that I has to select one of the available value from Units wile configuration and same will be part of field/attribute label. This is something which is not expected. As I said user should be allowed to choose value by themselves at runtime.
Can we do something to achieve this.