What is the Related Assets column in the Metadata .csv for?

Related Asset is there specifically to link two assets together (a cross reference) when there is something stronger than a tag required.

Some examples:

An image might have a related Usage License - so the Usage Licence PDF asset could be linked to the image assets it covers.
A Video could have a Shot List (Scenes description) Text asset linked specifically to it.

Related Assets are not for categorising assets - this is what tags, licenses and brands are for.

Generally Related Assets is rarely used - and we recommend you only set a up a Related Asset in the Admin interface (rather than in the CSV).