+ * Storage location of the base of this variable
+ *
+ * The precise meaning of this field depends on the nature of the variable.
+ *
+ * - Vertex shader input: one of the values from \c gl_vert_attrib.
+ * - Vertex shader output: one of the values from \c gl_varying_slot.
+ * - Geometry shader input: one of the values from \c gl_varying_slot.
+ * - Geometry shader output: one of the values from \c gl_varying_slot.
+ * - Fragment shader input: one of the values from \c gl_varying_slot.
+ * - Fragment shader output: one of the values from \c gl_frag_result.
+ * - Uniforms: Per-stage uniform slot number for default uniform block.
+ * - Uniforms: Index within the uniform block definition for UBO members.
+ * - Non-UBO Uniforms: uniform slot number.
+ * - Other: This field is not currently used.
+ *
+ * If the variable is a uniform, shader input, or shader output, and the
+ * slot has not been assigned, the value will be -1.