Glossary of Terms
CELL
GDSII and OASIS files are organized into groups of cells. At least one cell must be defined in the database. Cells are collections of entities such as boundaries and paths and text and references to other cells. The nomenclature cell is identical to structure which is a name derived from the GDSII specification. There is no order in which cells need appear in the GDSII file.
BOUNDARY
A boundary is a polygon defined by a minimum of 3 edges. The original GDSII specification limited the number of vertices to 200 but that limit has generally been ignored by most modern EDA software. Due to the nature of the GDSII record, there cannot be more than 8192 vertices per boundary or they would run out past the maximum record size. The interior of a boundary is always considered filled. The boundary edge may not self overlap but it may self touch under certain conditions.
PATH
A path is a series of connected line segments with a constant width. In principle, a path should not loop over itself but this is sometimes seen in data. The GDSII specification does not make any mention of corner mitering but many CAD systems automatically miter the corners where a path bends -- there is no standard rule for this mitering.  The original GDSII spec limited paths to 200 vertices but this limit is regularly ignored. Again, the actual maximum number of vertices is 8192 which is limited by the GDSII record size.
SREF
SREF is an abbreviation of Structure Reference. It is an instruction to place a structure (or cell) in a particular location. The placement can include scaling, rotation and mirroring. A cell may contain many SREFs to other cells but it may not reference itself and references may not be circular - i.e. if A references B then B may not reference A.
AREF
AREF is an abbreviation for Array Reference. It is an instruction to place a structure (or cell) in an array. The instruction includes the number of X elements, number of Y elements, X spacing, Y spacing, insertion coordinate and rotation and mirroring.
LAYER
A layer is a number assigned to an entity such as a boundary or path and conceptually links such entities into a common physical layer. It is an integer between 0 and 63  in the original GDSII spec but many EDA systems allow layers up to 4096. Cells and SREFs do not have a layer associated with them.
DATATYPE
A datatype is a number assigned to an entity such as a boundary or path and conceptually links such entities. It is an integer between 0 and 63  in the original GDSII spec but many EDA systems allow layers up to 4096. Cells and SREFs do not have a datatype associated with them.

The datatype can be used to subdivide entities on a common layer. For example, one may mark a boundary with a layer of 55 and assign it a datatype of 0. A second boundary on layer 55 may have a datatype of 1. This would allow the user to differentiate two boundaries on the same layer. The datatype assignment, in itself, has no particular universal meaning.

Qckvu can either ignore the datatype or treat it as a layer parameter.
HIERARCHY
Refers to the organization of cells/structures in a GDSII/OASIS file. Generally, a GDSII file is organized like a pyramid - at the very top stands the top cell which has references to other cells which refer to other cells. A cell at the very top has a level = 0. All cells references contained by a level 0 cell are assigned to level 1 and all cells references contained in a level 1 cell are assigned level 2.  One can choose to display only cells from certain references. This is often used to get a high level look at a database and to ignore data farther down the hierarchy. A database with only one cell and no references is called flat.