To configure a string enumeration field, set the field type to STRING_ENUMERATION:

Retrieval

Include the string enumeration field in the select clause:

select GENRE from ZBKS_BOOK

The retrieval field should be defined as follows:

<CustomRetrievalField>
  <FieldRef levelType="ZBKS.Book" category="PDF">
    <FieldName>Genre</FieldName>
  </FieldRef>
  <Type>STRING_ENUMERATION</Type>
  <EnumerationType>ZBKS.Genre</EnumerationType>
  <SimpleMapping sqlName="GENRE"/>    
</CustomRetrievalField> 

Filter

For the filter field, you can specify either a single-select or multi-select field. Here are examples of each:

Single-select

Provide the following in the WHERE clause of the SQL:

${filterIfNotNull:GENRE,genre = $GENRE$}

The filter field definition is as follows:

<CustomFilterField>
	<FieldRef category="PDF" levelType="ZBKS.Book">
		<FieldName>Genre</FieldName>
	</FieldRef>
	<Type>STRING_ENUMERATION</Type>
	<EnumerationType>ZBKS.Genre</EnumerationType>
	<SimpleMapping sqlName="GENRE"/>
</CustomFilterField>

Multi-select

Provide the following in the WHERE clause of the SQL. Note the difference from the clause used for the single-select variation:

${filterIfNotNull:GENRE,genre IN $GENRE$}

The filter field definition is the same as single-select, except there's an extra attribute uiFieldRenderMode which tells the UI to display it as a checkbox group:

<CustomFilterField uiFieldRenderMode="ColumnarCheckboxesNoButtons">
  <FieldRef category="PDF" levelType="ZBKS.Book">
    <FieldName>Genre</FieldName>
  </FieldRef>
  <Type>STRING_ENUMERATION</Type>
  <EnumerationType>ZBKS.Genre</EnumerationType>
  <SimpleMapping sqlName="GENRE"/>
</CustomFilterField>

To display a different column name for the string enumeration, provide the level type as "Undefined", with the desired column name in the field name:

<CustomFilterField>
	<FieldRef category="PDF" levelType="Undefined">
		<FieldName>TypeOfBook</FieldName>
	</FieldRef>
	<Type>STRING_ENUMERATION</Type>
	<EnumerationType>ZBKS.Genre</EnumerationType>
	<SimpleMapping sqlName="GENRE"/>
</CustomFilterField>

Following are the key points to note:

levelType in the FieldRef is pointed to the ZBKS.Book model, which is where the Genre field is defined.
category is set to PDF (Pre-defined field)
Type is set as STRING_ENUMERATION (Note the capital letters)
SimpleMapping is used to map the field with the SQL parameter
EnumerationType is used to define where the enumeration is mapped to
To display a different label for this field, set the level type as Undefined and populate the field name with the required label.

Following uiFieldRenderMode are supported for String Enumeration filter field:

ColumnarCheckboxes: Display the fiter field with checkboxes in 3-column layout (multi-select).
RadioGroup: Display the filter field with radio-buttons in 3-column layout (single-select).
EnumAutocompleteField: Display the filter field with auto-complete option (multi-select).