def to_value(unit_system: str, unit_category: str, str_input: str,
    str_ref_unit: str=None) ->float:
    """Convert a given input string into a float value. 

    :param unit_system: The unit system, from bpy.utils.units.systems. 
    :type unit_system: str
    :param unit_category: The category of data we are converting (length, area, rotation, etc.), from bpy.utils.units.categories. 
    :type unit_category: str
    :param str_input: The string to convert to a float value. 
    :type str_input: str
    :param str_ref_unit: A reference string from which to extract a default unit, if none is found in str_input. 
    :type str_ref_unit: str
    :return:  The converted/interpreted value. 
    """
    pass
