For some reason I can't use a prefix that contains an underscore.

$only['prefix'] works
$only['prefix_product'] does not work

any ideas?

What happens in that case? Any reason why you need the underscore anyway?

When using an underscore the category is not created, even after creating templates with that particular prefix. I can only create a category when not using an underscore.

I have multiple custom products and wanted to use an underscore for organization. For example, vba uses vba_links and vba_portal.

I have a feeling you are doing this wrong. If you have:

$only['yourname'] = 'Added by YourName';

Then, the templates you would add to go into that group would be named "yourname_template1" and "yourname_template2" or "yourname_this_that"

I understand this completely.

I am unable to do the following:
$only['yourname_product'] = 'Product';
yourname_product_itembit
yourname_product_whatever

I am unable to create a template prefix group that contains an underscore :(

Maybe it's a bug? If so, enter it into the bug tracker.

Do you have a prefix that is just "yourname", as well as "yourname_product"? That would probably create a conflict. However, if this is not the case, it does look like a bug.

Best is not to use underscores within the prefix as the underscore is used as delimiter.

You can use underscores but you will need to keep in mind the following:
- If the first part (xxx_yyy_templatename) xxx is used in multiple prefixes (not recommended!!) templates will be placed in the first group that matches the prefix.
- Changing the order in which the prefixes are added to $only does make a difference.
$only[test] = "Main testgroup";
$only[test_sub] = "Sub testgroup";

will give different result as:
$only[test_sub] = "Sub testgroup";
$only[test] = "Main testgroup";

Thanks for the info :)