Refer to figure.
ICAO Doc 8168
1.3 AREAS
1.3.1 Where track guidance is provided in the design of a procedure, each segment comprises a specified volume of airspace, the vertical cross-section of which is an area located symmetrically about the centre line of each segment. The vertical cross-section of each segment is divided into primary and secondary areas. Full obstacle clearances are applied over the primary areas reducing to zero at the outer edges of the secondary areas (see Figure I-2-1-2).

1.3.2 On straight segments, the width of the primary area at any given point is equal to one-half of the total width. The width of each secondary area is equal to one-quarter of the total width.

1.3.3 Where no track guidance is provided during a turn specified by the procedure, the total width of the area is considered primary area.

1.3.4 The minimum obstacle clearance (MOC) is provided for the whole width of the primary area. In the secondary area, MOC is provided at the inner edges reducing to zero at the outer edges (see Figure I-2-1-2).

We are interested in how the primary and secondary areas are established in this question. Be careful that primary area is not divided into two, it has a half of the total width in the center.