So, you’ve created a new custom Field (or column) type, but you don’t want it to be available to add directly to Lists and Libraries – that is, you want to force users to create columns as Site Columns, and add those to the list. Conversely, maybe you want to prevent a column being created as a Site column, and only available to add directly to lists (though I can’t thinik why).

Well, it turns out that that is possible. In the CAML defining your field in fldtypes_???.xml, you can have something like the following:

<FieldTypes>
    <FieldType>
        <Field Name="TypeName">MyCustomField</Field>
        <Field Name="ParentType">MultiChoice</Field>
        <Field Name="TypeDisplayName">My Custom Field</Field>
        <Field Name="ShowOnColumnTemplateCreate">TRUE</Field>
        <Field Name="ShowOnListCreate">FALSE</Field>
        <Field Name="ShowOnDocumentLibraryCreate">FALSE</Field>
        <Field Name="ShowOnSurveyCreate">FALSE</Field>
        <Field Name="Sortable">FALSE</Field>
        <Field Name="Filterable">TRUE</Field>
        <Field Name="ShowInEditForm">FALSE</Field>
        <Field Name="UserCreatable">TRUE</Field>

ShowOnColumnTemplateCreate controls visibility as a Site Column type. The other ‘Show On X’ values control visibility as a List Column Type for different types of List. You could, therefore, have a Column Type that can only be used on, say, Document Libraries. Hope that helps.