Hi Betsy I'm pretty sure you can use both categories as I have seen cards like that under both.

Hi Betsy,
 
Reviews have told me a number of times that relationship specific, age specific can only go under the relationship category.  See WIki http://www.gcuwiki.com/ig.html
It makes sense that the general category could really get bogged down with special relationship cards.
 
I usually do 1 of 2 things to get more exposure:
1. Create a readjusted matching card with no relationship and put it in the age specific category.
2. If card is for kids, I usually create a matching card and replace relationship with "to a special girl" or "to a special girl" then this can go under general birthday, age specific category.  It goes pretty fast if you are customizing the relationship.  A plus is that these more generic cards are more likely to be purchased by bulk card buyers.
 
Hope that makes sense.
 
Kati