The context of this question is the Form, Fit and Function (FFF) rule defined in ASME. It states that form is used to define any product with reference to its size, mass, shape, dimensions, and other visually identifiable features. For example "SCREW, PAN HEAD, M3 x 0.5, 2mm Lg, 316 SS". I am guessing this is Lag length based on some research i.e. the length from the head where the thread starts.
Q) What does Lg actually mean? Please quote resources for more reading