7.3.1 Sectional diagrams 

Where the figure is a cross-section on another figure, the latter should indicate the position and may indicate the viewing direction.

Each sectional figure should be capable of being quickly identified, especially where several cross-sections are made on the same figure, e.g. by inscribing the words "Section on AB", or to avoid the use of lettering, by marking each end of the cross-section line on the diagram with a single Roman numeral. This number will be the same as the (Arabic) numeral identifying the figure where the section is illustrated. For example: "Figure 22 illustrates a section taken along the line XXII‑XXII of Figure 21".