Generally three types of specifications are used i.e. technical, performance and brand name. Technical specifications state the physical and chemicals properties desired in the product. When the desired properties are in measurable terms they are called dimensional specifications. This type of specifications can be checked easily. Generally in case of oil paints and metallic raw materials this type of specifications is used. Sometimes materials to be used and method of manufacture are also prescribed. This is another form of technical specifications. This happens generally in case of military purchasing, because military purchases are made as per the needs of the defense departments, where standard specifications do not exist. Now the military requirement is a confidential matter and therefore the supplier/manufacturer has to follow the instructions and specifications provided by the defense department. In case of industrial purchasing, it is not so, unless and until the requirements of a particular unit are of special type or say unique. In case of industrial purchasing generally the supplier / manufacturer knows what the buyer wants or what performance he / she expects from the articles purchased and therefore the supplier / manufacturer uses the materials and methods of manufacture accordingly.
Performance specifications describe in words the performance or use of the items to be purchased or the services that the product will give. Here the buyer gives importance to the performance of the product and not to the analysis of materials used or the dimensions of the product or the components used. It should be noted that the conformance to performance specifications can be determined by use of the product, while that of technical specifications can be determined by tests in laboratory. The supplier / manufacturer is not advised about the type of materials or the process / method of manufacture to be used. The supplier is free to use materials and process/method of manufacture which he/she thinks suitable and he / she may take advantage of most up to date knowledge of technological development to provide the best product at the lowest cost. When a buyer gives a supplier free choice to select materials as well as the process / method of manufacture, the buyer must be sure of the supplier’s reliability and capability. Unreliable and incapable supplier may cause a great harm. A supplier, not fully reliable, may suggest another more expensive and better material than needed or he /she may not be able to supply the materials in required quantity. Only the reliable suppliers can supply the materials or products at reasonable prices and can therefore transfer the benefits of advanced technology to the buyers, because only they make use of advanced knowledge. Performance specifications are extensively used in purchasing highly technical military and space products and also complicated machines and machine tools. Sometimes buyers advise the suppliers to use specific methods or process of manufacturing and materials. Such practice is not advisable except in case of military purchases. Here the buyer becomes fully responsible for the performance of the products, so long as his specifications were met by the suppliers. Moreover, the benefits of advanced technology in form of new and improved methods of production etc., with which only the suppliers are familiar, may not be available to the buyer. Further, it is also very costly and time consuming to prepare specifications and to inspect for compliance. But when the buyers want to meet their specific needs or when the suppliers are too small to have research and development facilities, such practice can be adopted by buyers.
Now sometimes specifications take the form of blue print. The blue print is the most accurate form of description and is used when close tolerance or high degree of mechanical perfection is required. Its use is expensive not only because of he cost of the blue print, but also it usually describes an item of special design that is costly to manufacture. This method is the most feasible method of description for items for like castings, forging, tools, fixtures, punchings, machine parts etc where extreme accuracy is a must. Moreover, blue prints provide the basis for an accurate check on compliance, with specifications by the inspection departments, when material is received.