Log book / journal is used to note everything that is done during a project. There are two main purposes for this. Firstly to assist the project leader (and the project team if there is one). If things are not registered then it is easy to forget decisions taken months or years back in time. The second reason is to make after review possible. For the latter reason the log book / journal must be locked, meaning that everything is written in stone (can not be changed or deleted).