Usage of a few fields in marketing documents

Hi,

1. In AP Invoice, there are 2 fields where you can enter any arbitrary text: Free Text and Item Details.

Are there any guidelines when to use which one? There is in fact a 3rd field called Legal Text as well.

2. Similarly, there are 2 similarly named fields: Unit Price and Rate. but you can not enter Rate field. It is skipped.

What is the use of the Rate field?

3. For the Gross Price field, the Help File mentions that:

Enter in this field the unit price including tax. SAP Business One calculates automatically the unit price before tax and the tax amount, based on the tax code in the row. After pressing the 

TAB

The tax amount is calculated as follows:

Gross Price / (1 + Tax %) * Tax % = Tax Amount 

But the cursor does not even go to this field! How can I enter the unit price including tax?

Thanks.