The fm:Form_Custom element allows you to create your own form-fields, in the style as all the other form elements. Using it only makes sense when you're using fm:Form, because it's only purpose is to simply allow you to add new rows of fields in the same style as all the other form components.
This basically means if you specify formType="table" in your fm:Form component, this components will create a table-row and a <label> tag.
<fm:Form_Custom label="submit now"><button type="submit">Submit!</button></fm:Form_Custom>
|id||Optional||randomized||The id of the form element. Note that this id will also be used for the 'for' attribute in the <label> component|
|label||Optional||N/A||This attribute will be used to generate labels for this component, if it's used within an fm:Form component|
|language||Optional||N/A||This specifies which language to use for the display of the form_country dropdown. Choices are: |
|name||Required||N/A||The name of the form field. This is how the value will be submitted back to the server|