A reprint means more copies are being printed with no substantial changes. Perhaps a few typos are being fixed. A new edition means that there has been substantial change: content has been altered in a way that might make a customer complain that this was not the product that was expected. Or, text has been changed to add a new feature, such as a preface or appendix or additional content. Or, content has been revised. Or, the book has been redesigned.
No. A new edition is considered a different product and gets its own ISBN.
ISBNs are assigned to the volumes as they are sold as products. If they are only available as a set, the set gets one ISBN. If each volume is available separately and as a set, each volume gets an ISBN and the set gets an ISBN.
An ISBN is assigned to each book in the series. A series of books is also eligible for an ISSN (International Standard Series Number), available from the Library of Congress.
Yes, such reference works are routinely tracked by revenue strands (individual vs. library subscription reporting) by publishers.
ISBNs are assigned to the package and to the individual products in the package if (a) the product is eligible for an ISBN and (b) the products are sold separately. Any product that needs to be ordered as a distinct product and that is eligible for an ISBN, should be assigned an ISBN in addition to the package itself, if it contains items that are eligible for an ISBN.
ISBNs are not assigned to magazines, academic journals or other periodicals. However, if a single issue of a periodical is being sold as a book, then that issue alone may be assigned an ISBN.
The ISBN is intended for a monographic publication: text that stands on its own as a product, whether printed, audio or electronic. ISBNs are never assigned to music, performances or images, such as art prints or photographs. Consult this chart to determine if specific products/entities are eligible for ISBNs.