Neither matter nor highly-concentrated energy could normally penetrate a shield. When shields were "up," or energized at a high level, most matter or energy that came into contact with the shields was harmlessly deflected away. This was important in starship combat, as shields were essential for hull protection. When the shields were up, only minor hull damage would be expected during combat. In the 23rd and
24th century, without deflector shields weapons were capable of causing catastrophic damage to starship hulls almost immediately. (
Star Trek II: The Wrath of Khan;
Star Trek VI: The Undiscovered Country;
Star Trek Generations)
Continuous or extremely powerful energy discharges could progressively dissipate the integrity of a shield to the point of failure. Shield capacities varied according to many variables, from the power available to environmental concerns, making definitive and universal calculations of how much damage they could take difficult to estimate. Therefore, during combat tactical officers would continually report on shield strength, usually as a percentage of total effectiveness, with 100% meaning that the shields were at full capacity, and lower percentage scores indicating weaker shield conditions. Specific sections of the shield grid could take more damage than other sections, and be reinforced with additional power reserves, so tactical officers would report on the health of the shields by section if need be. Shields were said to be "holding" if damage was not sufficient enough to allow a compromise; if the shields were "buckling" or "failing," then a total loss of shield protection was imminent. (
Star Trek VI: The Undiscovered Country)