The Managed Custom Data Classes pages allows administrators to create or modify data classes used in custom inventory.
It is possible to create multi-rows data classes (data classes that will be able to have more than one entry per computer) however the implementation of the "Allow multiple rows from a single computer resource" button is a little confusing.
If you navigate to the Managed Custom Data Class you will see the following (with the left hand side showing your custom classes instead of mines of course):
This looks quite normal (given none of the existing custom data classes are selected) however if you switch to a custom class the check box will not be updated to show the state of the selected data class. So you cannot tell by looking at the UI whether a custom class allows multiple entires per computer or not.
To do this you have a couple of options: go to "Settings > Resource and Data Class Settings > Data Classes > Inventory > Custom". Select the class you are interested in and you will see whether it allows multiple entries.
Actually what is a little worse with the UI as it stands (I have SP5 installed with all but the CMDB updates in place due to a problem with the Install Readiness Check) is that if you click on the "Allow multiple rows" check box it will remain true for all the data class you check from then on, as the control is at the page level (so it doesn't get updated when the attributes are loaded in the UI grid...
There is a case I opened with the Inventory Solution team, but I suspect this is working as design right now...