The IfcRelProjectsElement is an objectified relationship between an element and one projection element that creates a modifier to the shape of the element. The relationship is defined to be a 1:1 relationship, if an element has more than one projection, several relationship objects have to be used, each pointing to a different projection element. The IfcRelProjectsElement establishes an aggregation relationship between the main element and a sub ordinary addition feature. 

The IfcRelProjectsElement implies a Boolean operation of addition for the geometric bodies of the element and the feature element. As with all decomposition relationships it determines:

• Existence dependency - the RelatedFeatureElement cannot exist without the RelatingElement
• Hierarchical and non-cyclical relationship - the IfcRelProjectsElement can only alter a single IfcElement
• No spatial containment - the IfcFeatureElementAddition as related element never participates in the hierarchical spatial containment relationship IfcRelContainedInSpatialStructure

In contrary the IfcRelAggregates relationship established an aggregation of equal parts to a whole. 

changes 

• IFC4 changes: Supertype changed from IfcRelConnects to IfcRelDecomposes.