Question

distinguish when price May or may not allowed

Answer 1

Price discrimination is a microeconomic pricing strategy where identical or largely similar goods or services are transacted at different prices by the same provider in different markets. Price discrimination is distinguished from product differentiation by the more substantial difference in production cost for the differently priced products involved in the latter strategy. Price differentiation essentially relies on the variation in the customers' willingness to pay and in the elasticity of their demand.

The term differential pricing is also used to describe the practice of charging different prices to different buyers for the same quality and quantity of a product, but it can also refer to a combination of price differentiation and product differentiation. Other terms used to refer to price discrimination include equity pricing, preferential pricing, dual pricing and tiered pricing. Within the broader domain of price differentiation, a commonly accepted classification dating to the 1920s is:

Personalized pricing (or first-degree price differentiation) — selling to each customer at a different price; this is also called one-to-one marketing. The optimal incarnation of this is called perfect price discrimination and maximizes the price that each customer is willing to pay.
Product versioning or simply versioning (or second-degree price differentiation) — offering a product line by creating slightly different products for the purpose of price differentiation, i.e. a vertical product line. Another name given to versioning is menu pricing.
Group pricing (or third-degree price differentiation) — dividing the market into segments and charging a different price to each segment (but the same price to each member of that segment). This is essentially a heuristic approximation that simplifies the problem in face of the difficulties with personalized pricing. Typical examples include student discounts and seniors' discounts.