There is no official definition of a series of postcards, but the essential characteristics can be defined.

Postcards printed or edited by the same publisher or printer and having identical characteristics are grouped into series. At a minimum, the publisher or printer or the characteristics of the series must be clearly identifiable in order to define a series.
The experience makes it possible to recognize the cards of the same series. Preferably the essential characteristic will be the printer before the publisher and if several publishers exist for a printer, we will define subsets specific to these publishers via sets of cards.
The most easily identifiable series are those which are numbered in a more or less orderly fashion.
The postcards were also sold and distributed in a pocket. We will speak of a "set" of cards. The set of cards is a coherent set of cards, for example illustrated by an illustrator or published for a specific place or event.
All sets of cards with similar characteristics will be grouped into series.
For ease of listing the cards we can divide a series into different sets even if these sets did not really exist commercially (no known cover for example). The existence of a pocket makes it possible to clearly identify a set of cards.
Please note that very large publishers have regularly revised their series and their numbering and listing can become very difficult and on the other hand some cards with the same number and the same title exist in different versions because after a number of copies the "negative" was worn or even broken and no longer allowed the impression of the original view
If in a series we find with the same reference cards with different pictures or titles they will be listed under two different cards
On the other hand, if only the typography of the title, the printing information or its position differ, several cards will not be listed because these are minor variations which do not impact the card search.
When there are no sleeves there is no fundamental difference between the set and the series: the characteristics of the series and the sets are almost the same. We prefer to define a set when it is a small set of cards and we will speak of series when there are very many cards with multiple subsets.