How to use Subworklfow is mentioned in above link which you have shared. Below is the explanation purpose of Subworkflow
Sub Workflow Improves readability and maintainability . Assume a scenario where you have set of activities which are common to more than one campaign then in that case you will create a subworkflow with that common activities(Logic) and you will call this sub worklfow form your main workflow and you dont have to build your logic again and again in every main workflows. you just need to call your sub workflow to do the logic part.
Below is workflow A which call sub workflow just after enrichment activity and i want rest of the work which is common to be build in subworkflow.
Now below is the subworkflow which having logic which I wanted to use to in more than one workflows.
1. Regarding Logs: Child logs are in the Master logs, labelled by subworkflow
2. In case of any error in child wf(subworkflow) , duplicate copy will be created , If no error Child sub-workflow in Being Edited status