HubSpot allows you to create custom properties to organize assets such as marketing emails, marketing events, and workflows. For example, here is a custom property that groups marketing events by the product that the event is about.
Marketing Events list with the custom Product multi-select property.
Unfortunately, these properties are not available to be used anywhere in HubSpot. They are only available on the page listing those records as an additional column and filter. Here are some places you might want to use these properites that you can't:
Lead Score. You can't filter marketing events or marketing emails or workflow enrolments by a custom property. (e.g. score any email with interaction = high as 2 points and emails interaction = low as 1 point based on a custom interaction level property.)
Workflows. You can't take workflow actions based on custom properties. (e.g. unenroll if enrolled in workflow with Type = followup based on a custom Type property.)
Segments. You can't filter records in segments based on these properties. (e.g. Attended a Marketng Event for the "Facilities" product based on the custom Product property.)
These properties are very interesting because they are visible and accessible to the user and are pitched as a great way to organize assets. Unfortunately, we can't access that organization anywhere except the asset's page, so it's usefulnessis severely limited.