--> this all are optional attributes which we can apply on filed in form view. if you want to find that which attribute we can apply on others then you can find from ".rng" file.

        <rng:optional><rng:attribute name="domain_filter"/></rng:optional>
        <rng:optional><rng:attribute name="attrs"/></rng:optional>
        <rng:optional><rng:attribute name="string"/></rng:optional>
        <rng:optional><rng:attribute name="completion"/></rng:optional>
        <rng:optional><rng:attribute name="width"/></rng:optional>
        <rng:optional><rng:attribute name="type"/></rng:optional>
        <rng:optional><rng:attribute name="ref"/></rng:optional>
        <rng:optional><rng:attribute name="eval"/></rng:optional>
        <rng:optional><rng:attribute name="search"/></rng:optional>
        <rng:optional><rng:attribute name="model"/></rng:optional>
        <rng:optional><rng:attribute name="use"/></rng:optional>
        <rng:optional><rng:attribute name="on_change"/></rng:optional>
        <rng:optional><rng:attribute name="domain"/></rng:optional>
        <rng:optional><rng:attribute name="filter_domain"/></rng:optional>
        <rng:optional><rng:attribute name="invisible"/></rng:optional>
        <rng:optional><rng:attribute name="password"/></rng:optional>
        <rng:optional><rng:attribute name="comparator"/></rng:optional>
        <rng:optional><rng:attribute name="sum"/></rng:optional>
        <rng:optional><rng:attribute name="bold"/></rng:optional>
        <rng:optional><rng:attribute name="avg"/></rng:optional>
        <rng:optional><rng:attribute name="select"/></rng:optional>
        <rng:optional><rng:attribute name="group"/></rng:optional>
        <rng:optional><rng:attribute name="operator"/></rng:optional>
        <rng:optional><rng:attribute name="colspan"/></rng:optional>
        <rng:optional><rng:attribute name="nolabel"/></rng:optional>
        <rng:optional><rng:attribute name="default_get"/></rng:optional>
        <rng:optional><rng:attribute name="required"/></rng:optional>
        <rng:optional><rng:attribute name="readonly"/></rng:optional>
        <rng:optional><rng:attribute name="view_mode"/></rng:optional>
        <rng:optional><rng:attribute name="widget"/></rng:optional>
        <rng:optional><rng:attribute name="context"/></rng:optional>
        <rng:optional><rng:attribute name="states"/></rng:optional>
        <rng:optional><rng:attribute name="digits"/></rng:optional>
        <rng:optional><rng:attribute name="icon"/></rng:optional>
        <rng:optional><rng:attribute name="mode"/></rng:optional>
        <rng:optional><rng:attribute name="img_width"/></rng:optional>
        <rng:optional><rng:attribute name="img_height"/></rng:optional>
        <rng:optional><rng:attribute name="size"/></rng:optional>
        <rng:optional><rng:attribute name="filename"/></rng:optional>
        <rng:optional><rng:attribute name="height"/></rng:optional>
        <rng:optional><rng:attribute name="rowspan"/></rng:optional>
        <rng:optional><rng:attribute name="align"/></rng:optional>
        <rng:optional><rng:attribute name="selection"/></rng:optional>
        <rng:optional><rng:attribute name="default_focus"/></rng:optional>
        <rng:optional><rng:attribute name="filters"/></rng:optional>
        <rng:optional><rng:attribute name="statusbar_visible"/></rng:optional>
        <rng:optional><rng:attribute name="statusbar_colors"/></rng:optional>