Resolution:
These "fixed" values will be saved into the repository as default values.
If your input XML document doesn't contain values for these attributes, when MB tries to get the AE schema of this element from the repository, it comes with these fixed values as default attributes.

For example,

You declare the following in the dtd file:

<!ELEMENT A (#PCDATA)>
  <!ATTLIST A value CDATA #FIXED "fixed val">

And your input XML document file is as follows:

&ltA&gtexample</A>

Then the generated AE Tree will look like:

A={
    @value="fixed val",
    ^CDATA^="example"
}

Therefore, the resulted xmlString from TreeToXML() will be:

&ltA value="fixed val"&gtexample</A>