Understanding Posting in Adempiere
What is the significance of Search Key in Accounts Element ?
other than searching of course
In reference to field C_ElementValue.Value
Search key can be used also to print on reports. And to represent visually the tree (although this is not enforced in any way)
What if I want to change the Search Key, after the system went live?
There is no problem until you don't define any external reference to Search Key.
Note that initially there is none foreign key referencing Values, only referencing _ID's, this is why external codes (search key's) can be easily changed by users without affecting functionality.