Create a custom field on the Transaction form (e.g. Sales Order) that would filter specific custom record entries created for another Transaction Record (e.g. Opportunity).

To do this, create a custom field that is a List/Record of Opportunity and set the Record is Parent box checked within the Custom Record Type. Then, use this Custom Record Type as List/Record on the Custom Transaction Body field and then add a Compare to Field= Opportunity as a filter.

For example:
I. Create a custom record type. 
    1. Navigate to Customization > Lists, Records, & Fields > Record Types > New.

    2. Give it a name (e.g. CR_Sample).
    3. Click Save. 
    4. On the Fields tab, click New Field button.
    5. On Label field, give it a name (e.g. CR_Opportunity).
    6. Type= List/Record.
    7. List/Record= Opportunity.
    8. Store Value= T.
    9. Record is Parent= T.
    10. Click Save.

II. Create a custom transaction body field with Type=List/Record of CR_Sample and then add a Compare to Field= Opportunity.
    1. Navigate to Customization > Lists, Records, & Fields > Transaction Body Fields > New.
    2. Give it a name (e.g. Custom field_CR_Sample)
    3. Type= List/Record.
    4. List/Record= CR_Sample.
    5. Store Value= T. 
    6. On Applies To tab, set Sale= T.
    7. On Sourcing & Filtering tab, fill out the Filter Using section using the options below. 
     • Filter Using= CR_Opportunity.
     • Compare Type= equal.
     • Compare to Field= Opportunity.
 
    8. Click Add. 
    9. Click Save.

Note:
- The only time that the Compare to Field will be giving out options such as "Opportunity" if we use Type= List/Record for custom field=   CR_Opportunity created within Custom Record Type= CR_Sample.  If a user is going to use Type= Multiple Select for this custom field= CR_Opportunity, the Compare to Field drop down menu is blank.

III. Create an opportunity record and a custom record entry.
    1. Navigate to Transactions > Sales > Create Opportunities. 
    2. Fill out the Opportunity form > Click Save. 
    3. On the Custom tab from the Opportunity record > On the Columns tab, click on  New CR_Sample button > Fill out the form to create a new custom record entry for that Opportunity record. 
    4. On the Sales tab from that Opportunity record, click on the New Sales Order button from the Closed Transactions subtab.

Actual:
- When a user looked at the Custom Transaction Body field named "Custom field_CR_Sample" on the Sales Order form, it only gives them the custom record entry created for that Opportunity record.