def _dim_arg(value, units):
    """Concatenate a specified units string to a numerical input.

    Parameters
    ----------
    value : str or number
        Valid expression string in the AEDT modeler. For example, ``"5mm"``.
    units : str
        Valid units string in the AEDT modeler. For example, ``"mm"``.

    Returns
    -------
    str

    """
    try:
        val = float(value)
        return str(val) + units
    except:
        return value