It's true that if you do a general person search on Ancestry, this database will be culled for your search criteria, but the results may be quite a few pages down the hit list. In general, if you think a particular database will be productive, it may be a good idea to search it specifically. To do that, click on the search tab, then select "catalog" and then enter words from the title or a key descriptive word.
OK, so after all that, what does "Registration Certificates" give you. Allow me to quote from Ancestry:
" The certificates in this database were generated by U.S. citizens living abroad who were registering their intent to retain their U.S. citizenship. The registrants were widows, divorced women, and minors who had been born abroad to parents who were U.S. citizens. Registrants used several different forms, including separate forms for women who had obtained U.S. citizenship via marriage and women who were citizens before marriage. Information contained on the certificate can include: name, birth date, birthplace, marriage date, spouse’s name, and spouse’s birthplace. Records may also list death date (spouse), marital status, residence, date a person left the U.S., dates of residence in foreign country, naturalization facts, and other details."