Comment 1 for bug 479645

The button is labeled 'Set' and
a) creates a new attribute with the name you entered into the text entry field 'Attribute name' if there doesn't already exist an attribute with the same name
b) updates an existing attribute with the name you entered into the text entry field 'Attribute name' with whatever value you added or modified in the 'Attribute value' entry box below.

related RFE: bug #170159 “XML Editor: editing in the list, multiselect, etc” <https://bugs.launchpad.net/inkscape/+bug/170159> includes among others the request to «… add: Button: "new attribute/property"»