The DS-5504 is the passport application used for a Name Change, Data Correction and Limited Passport Book Replacement Form. Having the correct information on your passport is very important to being able to use your passport.
If you changed your name and your old name is on your passport you can apply for a name change and will receive a new passport book with your proper name.
Rarely the State Department will have printing error with your passport and they will replace it for free. Occasionally an applicant makes a mistake when filling out the form and puts the incorrect information on there application, then when the passport is created it has the incorrect data. When this happens the passport holder must get a data correction application which is the DS-5504.
The DS-5504 is also used to extend the validity of a limited passport (if eligible). This will add a limited extension onto your passport validity date.