A gewaarmerkte kopie is a certified copy. When ordering a record, like your own birth record, you may be asked if you want a regular copy or a certified copy. Certified copies will be printed on special paper and will have a seal by the municipality guaranteeing authenticity.
For genealogical purposes, a regular photocopy will suffice. In fact, a regular photocopy can be more reliable for genealogical purposes than a certified copy since certified copies are sometimes extracted and retyped and creating the extract introduces the chance of copy errors. This is particularly the case if you ask for an international certified copy, in which case the record will be retyped in English. Regular certified copies in Dutch are usually certified photocopies, or print-outs from the civil registration database in case of modern records.
Because certified copies are created by clerks whose job it is to create accurate extracts, mistakes are rare. But any extracts can omit information that was on the original record, such as the information about deceased spouses and the names of the informants. So if you need a record for genealogical purposes, order a regular photocopy instead of a certified copy.
If you need the document for legal purposes, such as proving your identity to get married or apply for citizenship, you will need a certified copy. The fees for a certified copy will be higher than for a regular copy (2015: € 12.70). If you order a record, you should indicate what you need it for so the municipality will know what type of copy to give you. International certified copies are also available, which are created in English.