Line-items properties in Shopify typically represent item options, such as special instructions (e.g., the text for an inscription). These are automatically imported to ShipStation as part of the line-item's [Item Options] field, which can optionally displayed on Packing Slip Templates.
Sometimes, however, these are used for internal communications between e.g. Shopify apps. To support this, Shopify themes have a built-in convention where line-item properties prefixed with an underscore (e.g., _internalOption) are hidden from the customer:
Unfortunately, while these will be hidden on Shopify, they will be displayed on packing slips as part of the [Item Options] property (alongside valid customer options).
Ideally, ShipStation would either a) remove these when importing orders from Shopify, or b) hide these when displaying the [Item Options] field. (Acknowledging the latter would affect all customers, not just Shopify customers, and thus may be too invasive).