If set to True, multiple values can be stored in one field. The field should be of type list:integer or list:string. multiple references are handled automatically in create and update forms, but they are transparent to the DAL. We strongly suggest using the jQuery multiselect plugin to render multiple fields.
Note that when multiple=True, IS_IN_SET will accept zero or more values, i.e. it will accept the field when nothing has been selected. multiple can also be a tuple of the form (a,b) where a and b are the minimum and (exclusive) maximum number of items that can be selected respectively.