The numbering—in this case, "18"—indicates the prolific nature of these franchises. During the peak of physical media sales (DVDs), adult studios frequently released dozens of volumes within a single brand name to establish market dominance and brand recognition among consumers. These series often focused on specific tropes or "encounters" that emphasized spontaneity or amateur-style aesthetics, even when the productions featured professional performers.
Sometimes "Portable" referred to "best-of" collections designed to be a compact representation of the series. Why Volume 18? dukes hardcore honeys close encounters 18 a portable