http://rtfm.modx.com/display/ADDON/AjaxSearch#AjaxSearch-GeneralParameters
&whereSearch
Set in which tables & fields the search occurs. (optional)
tables & fields should be described as:
table_name[:list_of_field_names][|table_name[:list_of_field_names]]
The MODx "content_site" table is define as main table. Nick name: content
"tmplvar_contentvalues" table is defined as a joined table. Nick name: tv
"jot_content" table is defined as a joined table. Nick name: jot
"maxigallery" table is defined as joined table. Nick name: maxigallery
For these AS predefined tables, the fields available for search are the following:
content:pagetitle,longtitle,description,alias,introtext,menutitle,content
tv:tv_value
jot:jot_content
maxigallery:gal_title,gal_descr,gal_filename
All these fields are usable for display.
If you would like use the table for display but not as searchable fields, use the NULL metacharacter. e.g: &whereSearch=`content:NULL|tv`