Is only one summary task allowed for cost schedules and work schedules?

Original post: https://forums.buildingsmart.org/t/is-only-one-summary-task-allowed-for-cost-schedules-and-work-schedules/3526

Proposal: allow multiple top level cost items. There seems to be no technical reason to have a single top level cost item? If the user wants one, he can put one. If not, he doesn't have to.

Quoted below.

In IFC4, there is this statement:

> There is always a summary cost item as the root item of the tree representing the cost item nesting. Subsequent instances of IfcCostItem are assigned to the summary cost item using [IfcRelNests](http://localhost:8001/schema/ifckernel/lexical/ifcrelnests.htm). The summary cost item itself is assigned to [IfcCostSchedule](http://localhost:8001/schema/ifcsharedmgmtelements/lexical/ifccostschedule.htm) through the [IfcRelAssignsToControl](http://localhost:8001/schema/ifckernel/lexical/ifcrelassignstocontrol.htm) relationship.

Is more than one summary cost item allowed?

Option 1:

- Cost Schedule
- Summary Cost Item
- Nested Cost Item 1
- Nested Cost Item 2
- Nested Cost Item ...
- Nested Cost Item ...

Option 2:

- Cost Schedule
- Summary Cost Item 1
- Nested Cost Item 1
- Nested Cost Item 2
- Nested Cost Item ...
- Nested Cost Item ...
- Summary Cost Item 2
- Nested Cost Item 1
- Nested Cost Item 2
- Nested Cost Item ...
- Nested Cost Item ...

If it is option 1, what is the technical reasoning to have this summary task, to the point that it is even recommended that this summary task is "hidden" from the user?

Start with the linked buildingSMART forum discussion and the quoted IFC4 rules for IfcCostSchedule, IfcCostItem, IfcRelNests, and IfcRelAssignsToControl. Determine whether multiple top-level cost items are permitted and document the technical rationale or the specification change required; the issue mentions no repository files or tests.

Written by the indexing model from the issue text.