PCT International Search and Preliminary Examination Guidelines
4.28 The formal requirements relating to drawings are set down in Rules 11.10 to 11.13. The only question likely to cause difficulty is whether the text matter included on the drawings is absolutely indispensable. In the case of circuit diagrams, block schematics and flow sheets, identifying catchwords for functional integers of complex system (for example, “magnetic core store,” “speed integrator”) may be regarded as indispensable from a practical point of view if they are necessary to enable a diagram to be interpreted rapidly and clearly. However, such items can often be identified by a single numeral or letter which is then explained in the description.