Dear Cinthya,
Pl refer below HELP.SAP link which I believe will give you an idea of what actually those - System statuses for WBS Element mean (Lock / CLOSED).
Technically speaking, Set either status for WBS could depend on co-ordination between Projects / Finance team. For System status: Locked, you again can restrict individual business transactions or all the data in a work breakdown structure.
Hope the 'Features' section of the System status in below link will help you understand, how it works in reality during course of Execution of Project.
System Statuses in Work Breakdown Structures - Structures - SAP Library
-thanks