I have a form: Modalities, that is just a list of names basically, Modality_Names.  i.e. Massage, Acupuncture, Personal Training, Rolfing, etc.

Second form:  Company, in which there is a lookup field (Modalities_Selected) linked to Modalities and allows multi select.  i.e. Company.Modalities_Selected would be a list like Acupuncture,Rolfing

My third form:  Service_Providers, needs a field so they can only choose from the subsection specified by the Company.Modalities_Selected.  i.e. Service provider one chooses Acupuncture, Service provider two chooses Rolfing, and Service provider three chooses Acupuncture,Rolfing

My question is what is the best way to do this?  Use a lookup field and filter on the Service_Provider form, or just a multi select field and add each option, or another way I haven't thought of?

I'm not sure if the best way to do this, but this is what I came up.

Using a multi select field on the Service_Providers form and adding each of the Modality_Names as an option.  This occurs on user input for another lookup field to Choose your Company.

2. //When user selects their company from Choose your Company (Service_Providers_Company)
3. //fill the Modalities pick list so the Service Providers can only choose from those their Companies.Modalies_Selected
4. //get a list of the Modality ID's that the Company has chosen
5. mod_ID_list = (Company[ID == input.Service_Providers_Company].Modalities_Selected).getall();
6. //For each modality ID, Get the Modality Name from the Modalities form based on the ID
7. for each opt in mod_ID_list
8. {
9.     //Get Modality Name based on each ID in the list
10.     mod_name = Modalities[ID == opt].Modality_Name;
11.     //Add each Modality Name to the Pick list for the Service Provider to choose from
12.     SP_Modalities_Offered:ui.add(mod_name);
13. }
14. 
    

Let me know if clarification is needed.  Cheers.