Grade-of-Service (GOS) refers to the probability of a call not being able to complete due to a lack of facilities, in the case of a PBX this can mean trunks or time slots. The .001 is the ratio of calls completing to not completing. In this case it's blocking 1 call in a thousand. Depending on what you are trying to engineer you would take the GOS into account when you use the traffic engineeting tables, either the Retrial Table or the Erlang Table. Raising the GOS can get pretty expensive. You may have to add additional Port Networks or trunks to meet a GOS that only lasts for a few minutes a year.
Kevin
