Okay, I'll try to start again.
There is a registration form. The task is to be able to collect data on registrants in one of three ways -
By:
Category/Commodity or,
Procurement Professional or,
Purchasing Group
All data has been provided and is in a SharePoint list that I am pulling into a Collection.
This first image below is the rep collecting data by 'Category/Commodity'. These are all Combo Boxes pulling related data from the List or Collection.
Category Items:
Sort(Distinct(ServiceMaterialsDTECollection, Category), Result)
Procurement Professional DefaultSelectedItems:
With({wServiceMaterialsDTE:Sort(ServiceMaterialsDTE, CategoryManager, Descending)},Sort(Distinct(Filter(wServiceMaterialsDTE, Category in PrimaryMaterialsCategoryCB.SelectedItems),CategoryManager),Result))
Purchasing Group:
With({wServiceMaterialsDTE:Sort(ServiceMaterialsDTE, Title, Descending)},Sort(Distinct(Filter(wServiceMaterialsDTE, Category in PrimaryMaterialsCategoryCB.SelectedItems),Title),Result))

This second screen is the rep collecting data by 'Procurement Professional'. You can see the same Procurement Professional's name from above. She has '6' Category items associated with her and is associated with the Distribution Operations Buyers.

If I pick a second Procurement Professional the process (and formula shown below which is used in the 'Category/Commodity' combo box), shows that the '6 items' associated with the first member choice has been replaced by one item, 'Legal', associated with the second member choice, rather than having '7' items associated with both.
All items associated with 'all' names chosen in Combo Box one should appear in Combo box two.
The same should occur for 'Purchasing Group' (Combo Box three), based off the content of Combo Box one (which seems to be working; there is one Purchasing Group associated with each member.
Category/Commodity DefaultSelectedItems:
With({wServiceMaterialsDTE:Sort(ServiceMaterialsDTE, Title, Descending)},Sort(Distinct(Filter(wServiceMaterialsDTE, CategoryManager in PSMProcurementProfesionalCB.SelectedItems),Title),Result))
Filter(ServiceMaterialsDTE,Category, CategoryManager = PSMProcurementProfesionalCB.Selected.Result)

Purchasing Group DefaultSelectedItems:
With({wServiceMaterialsDTE:Sort(ServiceMaterialsDTE, Title, Descending)},Sort(Distinct(Filter(wServiceMaterialsDTE, CategoryManager in PSMProcurementProfesionalCB.SelectedItems),Title),Result))

The filter for 'Category/Commodity' above is (CategoryManage IS Procurement Professional):
Category/Commodity DefaultSelectedItems:
Filter(ServiceMaterialsDTE,Category, CategoryManager = PSMProcurementProfesionalCB.Selected.Result)
Also, when I try a similar formula to the other combo boxes, rather than the above I get a delegation error.
Error: Delegation warning. The "Sort" part of this formula might not work correctly on large data sets.
Category/Commodity DefaultSelectedItems:
With({wServiceMaterialsDTE:Sort(ServiceMaterialsDTE, Category, Descending)},Sort(Distinct(Filter(wServiceMaterialsDTE, CategoryManager in PrimaryMaterialsCategoryCB.SelectedItems),Category),Result))

Here is the Collection formula.
ClearCollect( RegistrantFormCollection,
{CompanyName: GCCompanyNameFld_1.Text,
PotentionalClassification: PotentialClassificationDD_1.SelectedItems.Value,
PrimaryCategoryType: PrimaryServiceMaterialRadioBtn_1.Selected.Value,
PrimaryCategoryCommodity: PrimaryMaterialsCategoryCB.SelectedItems.Result,
PrimaryProcurementProfessional: PrimaryMaterialsProcurementProfessionalCB.SelectedItems,
PrimaryPurchasingGroup: PrimaryMaterialsPurchasingGroupDD.SelectedItems,
PrimaryImage: Image2_1.Image,
PrimaryImmediateNeed: PSMCategoryCheckbox_1.Value
})