# IFC Structure

**URL:** https://forums.buildingsmart.org/t/ifc-structure/4215
**Category:** IFC (General Development)
**Created:** [May 4, 2022, 3:30pm UTC](https://forums.buildingsmart.org/t/ifc-structure/4215 "2022-05-04T15:30:34Z")
**Posts on this page:** 3
**Page:** 1

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### Author: ![Enrico](https://avatars.discourse-cdn.com/v4/letter/e/e495f1/32.png) [@Enrico](https://forums.buildingsmart.org/u/Enrico)
#### Post date: [May 4, 2022, 3:30pm UTC](https://forums.buildingsmart.org/t/ifc-structure/4215/1 "2022-05-04T15:30:34Z")

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Hello there

Are there any requirements that specify how the entries in an IFC (#1,#2,#3, etc.) should be listed?  
Is such a random structure in an IFC file allowed? I have seen already files with a random order.

```auto
#2 
#3
#5
#1
#4

```

Many Thanks!

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### Author: ![igor.sokolov](https://sea2.discourse-cdn.com/flex016/user_avatar/forums.buildingsmart.org/igor.sokolov/32/25_2.png) [@igor.sokolov](https://forums.buildingsmart.org/u/igor.sokolov)
#### Post date: [May 6, 2022, 12:42pm UTC](https://forums.buildingsmart.org/t/ifc-structure/4215/2 "2022-05-06T12:42:40Z")

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The order may be arbitrary.  
(this is under IFC - STEP specification)

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### Author: ![aothms](https://avatars.discourse-cdn.com/v4/letter/a/c67d28/32.png) [@aothms](https://forums.buildingsmart.org/u/aothms)
#### Post date: [May 8, 2022, 7:42am UTC](https://forums.buildingsmart.org/t/ifc-structure/4215/3 "2022-05-08T07:42:46Z")

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Indeed.

From 11.2 in ISO 10303-21 (2016)

> Each entity instance shall be represented at most once in the exchange structure and shall have an instance name that is unique within the exchange structure. **The entity instances need not be ordered in the exchange structure. An instance name may be referenced before it is defined.**

(emphasis mine)