When Main Line is true, Amount (Gross) is equal to Amount (Net) also equals Amount.
When Main Line is set to False in the Criteria tab > Standard sub tab, Amount is the Total Sales Order Amount while the Amount (Net) is Net of Discount only. This means that the Amount (Net) still includes the shipping and tax amounts.
The search is only able to capture the Discount Amount along the line items.
This is whether the Discount Item is added as a line item or applied to the whole order (under Items tab > Discount Item field).
This is currently being tracked in Enhancement 183081: Show values on Amount Discount column in transaction saved searches even when criteria is the following: Type=Sales Order and Main Line = True.
1. Amount (Net) is equals to Gross Amount Or Invoice Amount if Main Line =T. However, If Main Line=F, it is equals to the Invoice Amount less Discounts only.
2. Amount (Net of Tax) is equals to Invoice Amount less Tax if Invoice transaction is taxable but it is Null if the transaction is not taxable.
The Amount (Shipping) field extracts both the shipping and handling as a total.