Some creatures have been given a '0' for certain characteristics which means that they have no ability whatsoever in that field! This usually applies to creatures who are unable to use missile weapons, and so they have a BS of 0, but it might equally apply to other characteristics too. For example, a defenceless civilian will have no Attacks whatsoever.
0 Level Characteristics
Source: Core Rulebook 3E, p. 38
A '0' in a characteristic means a creature has no ability whatsoever in that field.