The field "Field: xxx" was not used in any charts or master items. This field could likely be dropped from the data model with no loss of analysis. Dropping this field would recover nn bytes.
The field was not found to be referenced in any resource -- Chart, Variable, Bookmark, etc -- or used as a key field. Removing the field from the script would reduce memory requirements by the indicated number of bytes.
It may be convenient to first use a script DROP FIELDS statement to remove fields and then validate you application. A "Drop Fields Dialog" can be found in the Tools menu. This menu will generate a filtered DROP FIELDS script statement for you.
There are some indirect "uses" of fields that QSDA can not detect such as:
•Fieldnames as a string list, such as may be used to populate a set of dropdown choices to feed a variable. e.g.
After dropping fields, it is recommended to run a QSDA collection to see if any syntax errors or "missing" flags have surfaced.