What's the "line-num"-field in Quotes?

Hi

I'm stuck on the line-num field in the Quote interface.

Can it be fetched using the api or is it calculated on the spot.

I found it in the metadata under the name "line_num"(modules.Products.views.quote-data-group-list.meta.panels[0].fields[0]):

{
"name": "line_num",
"label": null,
"widthClass": "cell-xsmall",
"css_class": "line_num tcenter",
"type": "line-num",
"readonly": true
}

I cannot find anything in the documentation about its type or the field itself.

Kind Regards