The \[i2b2 star schema https://community.i2b2.org/wiki/display/IDRT/131.+Background\] in its initial setup defined a one fact per concept per patient approach to a medicial data warehouse design. The central i2b2 table \[obsercation_facts\] is a enhanced EAV (entity--attribute--value) structure with  additional entity identifiers (patient, visit, provider). The visit dimension defines a (financial) encounter. This allows for the enhancement of the one-to-one relation (concept to fact) to a one-to-many relation by defining N financial encounters per patient. But source data often does not contain financial encounters and/or a fact bundling of much finer entities is needed.
With the addition of modifiers in i2b2 version 1.6 (see: Data Model of Modifiers in i2b2  https://community.i2b2.org/wiki/display/DevForum/Modifiers+in+i2b2+Data+Model) a more useful functionality for a undefined (meaning: not just financial encounter bundled) one-to-many relationship was introduced. Modifiers offer  a way of bundling concepts in in the i2b2 hierarchie and give these concepts a unique (for that patient and encounter) subidentifier in the observation_fact to connect facts of one patient. 
The [i2b2 star schema|IDRT:131. Background] in its initial setup defined a one fact per concept per patient approach to a medicial data warehouse design. The central i2b2 _table obsercation_facts_ is a enhanced EAV (entity--attribute--value) structure with  additional entity identifiers (patient, visit, provider). The visit dimension defines a (financial) encounter. This allows for the enhancement of the one-to-one relation (concept to fact) to a one-to-many relation by defining N financial encounters per patient. But source data often does not contain financial encounters and/or a fact bundling of much finer entities is needed.