When utilizing TitleLeaf, the term Set should not be confused with the term Series. TitleLeaf follows closely the ONIX for Books standard. To paraphrase their definitions, a Set is a salable product, composed of a fixed number of sub-products. A Series is not a salable product; it is a grouping for marketing purposes of an indefinite number of products published over an indefinite period of time.
In TitleLeaf terms, a Set is managed via the Product module (see Admin → Catalog → Products). A Set is a Product record with its type defined as Set (as opposed to Title). A Series, on the other hand, is managed via the Series module (see Admin → Catalog → Series). Another distinction is that a Series cannot span products from multiple publishers, whereas a Set can.
Further Explanation
In practical terms, a Set is snapshot of a Series on a specific date. The contents of a Set are fixed. If titles are added to a Series at a later date, then a new Set should be created to replace the old Set.
For an exact definition, please consult the following excerpts from ONIX for Books, Release 2.1, Revision 03:
PR.5 Series
A “series” means an indefinite number of products, published over an indefinite time period, and grouped together under a series title, primarily for marketing purposes. A series does not have an EAN-13 number, ISBN or UPC, and it is not traded as a single item, although it may be possible to place a standing order for successive items in the series to be supplied automatically.
PR.6 Set
A “set” means a finite number of products grouped together under a set title. The products may originally be published over a period of time, but generally they have become or will become available for simultaneous purchase. A set may be traded as a single item or in separate parts or both. If traded as a single item, a set should have its own product identifier such as an EAN-13 number and/or an ISBN.