Field Qualifier is accessible from Products Menu under Setup.
Field qualifier is used to validate lot number for lotted inventoried product, serial numbers, or custom field for lots.
To create a new field qualifier, you would need to provide a name, a description and a regular expression.
Regular expression is used to define a partter and to ensure the lot number, serial numbers etc matches a certain format.
For example, to allow any letters and numbers, you would use .* . This is used by most users to just allow anything format.
To allow just numbers, you can use this [\d]+ . \d means numbers.
To allow only alphabetic letters of any length, you can use this [:alpha:]+ .
To see more definition about regular expressions, see wikipedia.