>2. The example working but the principle not enough clear for me.
>Can you help me?

Searching one of my projects for "eval", I find additional examples:

1) Get the expression for an index. This can help you check for duplicate values, or check for referential integrity before VFP does.

? key() && gets the index expression for the current index
? eval(key()) && gets the value that is saved in the index.
* Searching for this value can be used to check for duplicates.

2) For my Text Report (see download #9991), I used eval() to check when a break expression changes (so that the report prints headers and footers when an expression changes).

3) Get the value from a field when the fieldname is stored in a variable. Example:

MyField = "ArticleName"
? MyField && shows: "ArticleName"
? eval(MyField) && shows the content of the field ArticleName