You can filter the items returned from the ERP/Accounting System by using the Item Filter in the Item Settings Screen.

For example, if you want to return all items except for the Widget27 item you can set the field to be  ITEM.Where(ItemCode != "Widget27")

The filter can be any valid lamda expression.

The Item Filter can also be used to modify the URL for getting the items from the accounting system by using ACCSYS. at the start instead of ITEM. 

For example: for Cin7 Core we can use the item filter to get the items for a specific channel by putting in ACCSYS.channels like '%A%' or channels '%B%'

This will bring all the items that have A and B in the channel name.