Inocentral, a Poser figure is composed by several files. There is the geometry file which describes the set of vertices and faces that create the base geometry. There are CR2 and PZ2 file which define the joints, bones, materials and poses. All these files are collected inside a pretty well defined directory structure. 
From a "root" directory you will find a sibdir called "Runtime". Underneath you will find directories with names like "Geometries", "Library" etc. 
Those directories define what is referred to as the Poser Runtime. DAZ Studio reads that structure as well. When you install a figure you add files to that repository/library/runtime (all valid names).
Take a look at it with your file manager of choice and read the documentation. It's overwhelming at first but after a bit of frustration it generally starts to make sense :)

Paolo

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