IFC 4.3.1.0 (IFC4X3_ADD1) development

# 5.3.3.5 IfcRelSequence

## 5.3.3.5.1 Semantic definition

IfcRelSequence is a sequential relationship between processes where one process must occur before the other in time and where the timing of the relationship may be described as a type of sequence. The relating process (IfcRelSequence.RelatingProcess) is considered to be the predecessor in the relationship (has precedence) whilst the related process (IfcRelSequence.RelatedProcess) is the successor.

IfcRelSequence is defined as one-to-one relationship; therefore it assigns one predecessor to one successor.

Use definitions

IfcRelSequence is used to describe the logical sequence relationship that exists between two processes. This logical relationship identifies that there is a predecessor or relating process and a successor or related process. In IFC, there may be one predecessor and one successor in the relationship. Many occurrences of IfcRelSequence may exist to describe the sequence relationships of a predecessor task with many successor tasks or of many predecessor tasks with one successor task, thus enabling a m:n sequence relationship between tasks. Please note that sequence relationships can be used to define dependencies between process occurrences but also between process types (for further information see IfcRelDefinesByObject and IfcTaskType). In case of defining dependencies between process occurrences sequence relationships should stay within the limits of a directed, non-cyclic graph.

A sequence type may be set for a sequence. For tasks assigned to a work schedule, it is expected that the sequence type will be asserted. For a process map, where the sequence relationship between processes is simply a logical flow, it need not be asserted.

A time lag may be assigned to a sequence, and the sequence type defines the way in which the time lag applies to the sequence either as a ratio or percentage of time duration (e.g. start successor task when predecessor is 50% complete) or as a time measure (e.g. start successor task 1 week after commencement of the predecessor task). Care should be used when assigning a time lag to a sequence depending on the setting of the sequence type since there is no checking that the time lag value is in keeping with the sequence type set.

## 5.3.3.5.6 Formal representation

ENTITY IfcRelSequence
SUBTYPE OF (IfcRelConnects);
RelatingProcess : IfcProcess;
RelatedProcess : IfcProcess;
TimeLag : OPTIONAL IfcLagTime;
SequenceType : OPTIONAL IfcSequenceEnum;
UserDefinedSequenceType : OPTIONAL IfcLabel;
WHERE
AvoidInconsistentSequence : RelatingProcess :<>: RelatedProcess;
CorrectSequenceType : (SequenceType <> IfcSequenceEnum.USERDEFINED) OR ((SequenceType = IfcSequenceEnum.USERDEFINED) AND EXISTS(UserDefinedSequenceType));
END_ENTITY;

## 5.3.3.5.7 References

Is this page difficult to understand? Let us know!

### 5.3.3.5.8 Changelog

#### 5.3.3.5.8.1 IFC4

• attribute, UserDefinedSequenceType
• where rule, AvoidInconsistentSequence
• where rule, CorrectSequenceType
• where rule, WR1
• attribute SequenceType optional, Is now optional
• attribute TimeLag optional, Is now optional
• attribute TimeLag type, Changed from "IfcTimeMeasure" to "IfcLagTime"