From In-Portal User Manual

Brackets

On this tab, the administrator can specify the brackets for the shipping type. See the shipping type description on the General tab for more information on what these brackets represent.
The mechanism of editing the brackets is very similar to editing price brackets in the Product edit form (tangible products). For a complete description, see smanual.
To add more brackets, use the “More brackets” button, which adds them in batches of five. To specify that the last bracket is open-ended (no limit), use the Infinity button. To arrange the brackets in ascending order after modifications, use the Arrange button.
In every bracket, you can only edit the ending amount, and the beginning amount is taken from the previous bracket. The first bracket, the beginning amount is zero. This is enforced by the system to avoid the gaps in the brackets and the ambiguity in the calculations. If the order base amount exceeds the maximum bracket (it’s possible when the last bracket is not set to infinity), the shipping will be considered impossible for this type.