var custpartnum = "";
custpartnum = currentRecord.getSublistValue({sublistId: 'item', fieldId: 'custcol_scm_customerpartnumber', line: i});

Which printed on the xml is retrieved by: 

<#assign custpart=key.custpartnum> ${custpart}

I wonder where I should link to the item name field (Name on the custom record)

probably need to share more code/template

Of the print script or the xml template ?

both

Can’t paste code on a conversation I think

it looks like the template is using a custom data source, which means you need to add the data to the object used in TemplateRenderer.addCustomDataSource

I did that, I can retrieve the ID, but would like the name of the record instead of the ID.

use Record.getSublistText

instead of currentRecord.getSublistValue, I use getSublistText ?