What does enctype="utf8" mean in HTML form?

Question

According to e.g. What does enctype='multipart/form-data' mean? there are 3 possible arguments for the enctype attribute in an HTML form:

• "application/x-www-form-urlencoded" (the default)
• "multipart/form-data"
• "text/plain"

But I have also seen enctype="utf8" in some forms. Is it common to use this too, and how does it relate to the 3 possibilities above?