All harassment is serious, but I guess when it comes to classification of minor versus serious, it leads more to what are appropriate measures to be taken, how extensive an investigation needs to be conducted, and what the severity is, potentially, of disciplinary measures down the road.
For example, if somebody came to me and said my colleague made a joke that could be interpreted as a racist joke, I would recommend in a situation like this that there be a one-on-one dialogue, potentially, with the assistance of an informal conflict resolution professional to try to resolve that issue at that stage.
However, if there was an incident where one employee made severely racist remarks on more than one occasion to another, then clearly a one-on-one dialogue, an informal conflict resolution, is not going to resolve the situation. That would require an elevated, more detailed process.