What is the difference between cost and sales?

As practised in the Project Management, the cost of project activities represent the planned or actual expenditures of money involved in carrying out tasks.
The sales is outside the system and handled through invoicing process with an input from Contract value, the Project progress value minus deductions.
This is how interim sales invoice are generated and claimed.
Both terms mean different things.

Thanks for your valuable answer.
Generally sales = cost + margin.
So in product sales shall b calculatd based on the production cost plus the margin.
But in projects,contract value shall be considered as a sales value but when we are claiming the bill / invoice to the client based on the contract/ bbu.
In this situation what we are claiming from client is one of the part of total contract value so shall v considered this as a sales.
Pls clarify

Cost is the total expenditure, including direct and indirect, incurred in production whereas sales is the revenue from a product which is generally cost plus profit.
A Product sales involves the revenue of certain units; but revenue from construction is stage-wise progression of billing as defined in the contract.
Contract or Project revenue involves deduction of retention whereas product sales is full price of units sold.

I understand the question intends to differentiate between Cost & Price.
Cost: Value of all resources that are required to produce an activity/ product/ asset.
Price: It is the monetary value at which something (product/asset) is being sold or being bought.
I.e.
exchange of ownership occurs between willing buyer and willing seller, in free market, under no compulsion and both are aware of all facts.

A vender quotes me his prices of an item70$ (This is my costt) when im gona trade this product to any of my client then70$ is my cost + my company overhead & profit is20$ + my expenses (fixed cost + variable cost) incurred in this sale is10$.
Now the selling price to my client is100$.