# Geometrical Tolerances--Separate, Combined or Simultaneous?
## 1 Introduction  
Since 1996, when Technical Committee ISO/TC 213 _Dimensional and Geometrical Product Specifications and Verification_[1] was established, it has been conducting extensive work to develop and improve standards that address geometrical tolerances [2, 3]. The aim is to provide contradiction-free rules and operators that allow designers to clearly and completely describe the acceptable variations of geometrical product characteristics such as size, orientation, location and run-out [4, 5]. The capability of the existing tolerancing tools given in the ISO GPS system standards (ISO Geometrical Product Specifications system standards) is crucial both for classical machining and forming as well as for modern production methods such as additive manufacturing [6, 7]. The ISO/TC 213 works are boosted by industries, mainly automotive and aerospace, that need indications to uniquely define how far the geometry of a manufactured part may be away from the nominal theoretically exact geometry and, when assembled, still operate according to the functional requirements.  
The graphical symbols for 14 geometrical tolerances (characteristics symbols) are given and explained in ISO 1101:2017 [8]. The rules for geometrical specifications for integral and derived features, using the line profile and surface profile tolerance symbols, are detailed in ISO 1660:2017 [9] and in ISO 5458:2018 [10] are given complementary rules to ISO 1101:2017 regarding pattern specifications as well as rules to combine individual specifications. The pattern specification is a combined requirement, indicated by a set of geometrical specifications and controlled by a combination of more than one tolerance zones having, without priority between them, constraints of orientation and location or only constraints of orientation [10].  
The 14 geometrical characteristics, with application rules defined in the standards listed above, are supplemented by the letter indicators given in these standards. Symbols such as CZ (Combined Zone), SZ (Separate Zones)--in the ISO GPS system, it is the default condition [11] and SIM (SIMultaneous requirement) were implemented at differenttimes (e.g., in subsequent editions of ISO 1101) or in various standards. These letter symbols determine the meaning of the geometrical characteristic specified in the first section of the tolerance indicator, which is crucial when writing a measuring program for a coordinate measuring machine to verify a manufactured part's conformance with the particular specification.  
It is underlined in [12] that geometrical variation management strongly relies on the gathering, processing, sharing and disseminating of tolerancing information that still requires many manual efforts and interventions. This paper aims to enable a complete understanding of what constraints are set when the CZ modifier is specified for a particular tolerance indicator and give recommendations for indications that secure univocal specifications. This is in line with the expectation that a reduction in the annotation contradictions, annotation redundancy, missing annotations and annotation uncertainty in the geometrical tolerances in Model-Based Definition (MBD)designing is the key to realising the digital integration of design, manufacturing and inspection [13].  
The results of interesting research regarding the implementation of statistical tolerance analysis in the German industry are presented in [14]. Among other conclusions, the authors write that it is undisputed that revisions to and extensions of the ISO GPS system standards lead to a better and more accurate tolerance design as well as that the ISO GPS standards are met in the industry with great reluctance and scepticism due to their immense scope and complexity. Taking all ove the above into account, the purpose of this paper is to clarify issues with the univocal selection and functionally relevant tolerancing of a pattern of features.