# Geometrical Tolerances--Separate, Combined or Simultaneous?
## 4 Conclusions  
The ISO GPS system indications are important tools that help engineers communicate the tolerances of a product when designed. The indications shall clearly express all the design requirements and be precise to secure the consistent interpretation of the engineering drawings. The ambiguity in the specification may be due, inter alia, to the absence of provisions in the standards or may be due to the incorrect extrapolations/interpolations of the examples of the specifications given in the standards. Some erroneous specifications have become entrenched over the years during vocational training and lectures at universities as well as industry practices and are, unfortunately, constantly being repeated. A pattern of holes or planes is quite often used as an assembly feature on a part. So the univocal tolerancing of such a set of features is a crucial designer task. According to the abovementioned issues, it is not recommended to specify a tolerance zone pattern for a group of features with a form or orientation indicator because the meaning of such a specification for a pattern is unclear. The form and orientation tolerances do not the fix linear theoretically exact dimensions between tolerances features. Therefore, they do not univocally distinguish the patterns of features. Only a location tolerance with a CZ modifier and a relevant datum or datum system (if required) unambiguously defines the constraints in a pattern. Therefore, it is strongly recommended to use position (symmetry) tolerance when a pattern of features is functionally required. In the last decade, Technical Committee ISO/TC 213 _Dimensional and Geometrical Product Specifications and Verification_ endeavours to eliminate the problem of the ambiguity in specifications by applying a new approach to standards development. Each new standard or revised edition of the standard that addresses geometrical tolerancing is rule-based. It contains a set of rules that generically define how the tolerances shall be specified and what their meanings are. Examples of the application of the rules formulated in each new or revised standard are given for a particular standard in its informative annex. Such an annex helps a user understand the rules given in the standard. On the other hand, the generic rules enable a user to indicate the tolerances for other cases according to the specific functional requirements. The great fundamental challenge for ISO/TC 213 experts is the effective formulation of rules in the standards, i.e., unambiguously and in a way that is easily understandable by all standard users. Figure 21: The position tolerance with the CZ modifier uniquely indicates that two toleranced axes together shall be perpendicular to datum A—the recommended specification that shall be used instead of the specification shown in Figure 19.  
It is worth understanding that some of the inconsistencies and contradictions highlighted and discussed in this paper are the subjects of vigorous dispute in Technical Committee ISO/TC 213. [PERSON], who works for the automotive industry, gave significant input [16]. Unfortunately, ref. [17] is an internal document of the ISO/TC 213 Working Group WG 18 _Geometrical Tolerancing_ and is hardly available. Each measurement requires the univocal definition of the measurand--the quantity that is intended to be measured. The rules and the modifiers provided in ISO 5458:2018 give designers new powerful tools to combine the individual specifications for geometrical features to create a pattern of features grouped with an external or internal location and/or orientation constraints. It should be emphasised, however, that some of the examples of pattern specifications given in Annex C (informative) of this standard need to be verified and corrected so that all constraints result only from the direct interpretation of the indication used and not from adopting additional assumptions that are not based on the rules given in the ISO GPS system standards.