What is the defining characteristic of unilateral tolerances?

Unilateral tolerances are defined by the characteristic that deviation from a specified dimension is only allowed in one direction. This means that the dimension can either increase or decrease, but only by a specified amount in one specified direction. For instance, if a part is specified with a dimension of 10 mm with a unilateral tolerance of +0.5 mm, it can measure anywhere from 10.0 mm to 10.5 mm, but it cannot go below 10.0 mm. This approach simplifies manufacturing and quality control processes by clearly defining acceptable variations, ensuring parts meet functional requirements without ambiguity.

In contrast to this, options suggesting variations allowed in both directions include deviations that would fall under bilateral tolerances, which permit a specified range of variation both above and below the nominal dimension. The option implying no deviations allowed depicts a scenario of exact tolerances, which is not characteristic of unilateral tolerances. The option that states only positive deviations are allowed would imply a specific limitation without recognizing the broader context of unilateral tolerances, which are purely focused on one-directional deviations.