Procedural details and requirements for kosher certification are available on the web-sites listed below. There are of course many others although not all kashrut authorities have web-sites. The "kosherness" of a hechshered product is not absolute; its formulation and manufacture will conform broadly to the appropriate Jewish laws which govern kosher foods but some kosher certifiers are more stringent while others tend towards leniency in the interpretation of those laws.
If you are intending to apply for kosher certification of your product then the stringency/leniency of a certifying agency may be important in the market where your product will be offered for sale. Retailers may select products whose hechsher is already known and respected amongst their customers.
If your product is, say, a chemical, e.g. an emulsifier, colouring or preservative, which will be sold only to a manufacturer as an ingredient in a kosher product then it's still worth checking whether the manufacturer's kosher certifying agency will be satisfied with your product's kosher certification.
In the list below the basic web-site address is shown plus the italicised name of the document, if the latter is known. However, if you click on an address then the full, often lengthy, address will be used.
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