Explanation
Campaign hierarchy is a feature that allows administrators to organize campaigns into a parent-child relationship, where the parent campaign represents a larger initiative and the child campaigns represent smaller or more specific activities within that initiative. Using campaign hierarchy can help Ursa Major Solar report on campaign ROI by month and series by creating a parent campaign for the series of networking events and creating individual child campaigns for each monthly event. The parent campaign can show the aggregated metrics and ROI for the entire series, while the child campaigns can show the metrics and ROI for each month.
Adding different record types for the monthly event types, creating individual campaigns that all have the same name, or configuring campaign member statuses to record which event members attended are not solutions for reporting on campaign ROI by month and series; they either do not group campaigns into a hierarchy or do not track campaign metrics or ROI. References:
https://help.salesforce.com/s/articleView?id=sf.campaigns_hierarchy.htm&type=5