How are split lines configured?

I work for a company where it is very common that we partially ship goods to customers.  For some items, when we partially ship a line, the sales order line is split such that a totally new line is created (in image below, line 1.000 was split to line 3.000).  However, for other items, the same line is maintained but a new decimal line is created (in image below, item 4.000 was split to 4.100).  We would like to have them all split to a decimal line number on partial shipment.