These directives can be used to define a structure - a consecutive group of variables with a fixed format.
You may only use .var and conditional directives inside a structure's definition. You can nest structures - for example:
.struct Point3D .var Point2D, P .var db, Z .endstruct .var Point3D, You ld a,(You.P.X)
You may not, however, nest a structure onto itself. You might feel tempted, for example, to do the following:
.struct TreeNode .var word, Value .var TreeNode, RightBranch .var TreeNode, LeftBranch .endstruct
The problem is that that particular structure has an infinite size! If you wish to implement that sort of data structure, you'd use pointers to the branch nodes.