The PLUS system is a modular concept for Eurorack enclosures which are based around 42HP wide PLUS-Modules as lowest common denominators. A PLUS-Module can be used alone or connected horizontally to other PLUS-Modules to form wider "boats" (84HP, 126HP, 168HP and so on).
Using the Row Expansion Pack (4 x 42HP) it is even possible to connect PLUS cases vertically to form enclosures with more rows.
The enclosures' sides can be closed with wooden side cheeks; per row one set of cheeks is required.
About the product:
The four PLUS modules are solid aluminium "boats" with 42HP width and a maximum internal depth of 43mm. They have integrated holes for pins and screws for connecting to other PLUS' and/or to wood sides as well as profiles for sliding M3 nuts.
The PLUS modules contained in the Row Expansion Pack differ from the individually available PLUS modules: on one long side four holes for screws are located which are used for horizontal mounting. Besides that on the same side there are longitudinal slots which are intended for feeding through "flying bus boards". This way you don't have to purchase one power supply per row but can use a single strong one for the entire case (e.g. a SILTA by FRAP Tools).
When purchased individually the Row Expansion Pack is ideally used in a 2x2 configuration i.e. two modules each are connected vertically and horizontally. This creates a a84HP wide case with two rows.
Used as an expansion you should already possess two standard PLUS Modules. This way you can create a 2 x 126HP or a 3 x 84HP configutaion.
FRAP Tools' website descriptively shows various combinations on PLUS modules and Row Expansion Packs.
single PLUS Module: 213 x 130 x 50mm (W x D x H)
four PLUS Modules in 2x2 configuration: 427 x 270 x 50 mm (W x D x H)