Is a personalized written letter required also?

I have found forms online to fill out all required information for an invitation letter, but does my husband (who has the work visa) need to make a personalized written letter requesting that my parents come over (something like a cover letter for a resume) even if all information is on the invitation letter form, and if they have copies of all required documents?

And which birth certificate is required, my birth certificate? It doesn't have my parents listed on it. Thank you!