Set the parameter value and meaning of ajax before sending setRequestHeader () in post mode?

there is a problem with learning ajax. You need to set up

before sending ajax in post mode.

ajax.setRequestHeader("content-type","application/x-www-form- urlencoded");

but I heard that the second parameter also has "multipart/form-data" , "text/plain" these values;

what do these values mean?
what type of data can be transferred in post mode? Does it have an impact on different values? Is there any other value for
?

look up the information everywhere but can"t find it, find the answer

Javascript ajax

Mar.21,2021

you can take a look at Multi-purpose Internet Mail extension (MIME) types

in fact, the server often does not judge these types, but deals with them directly according to the interface definition.

you can transfer strings. If it's get, you can write null, in send () without writing it directly, but if it's post, you have to write an address or spell it after it. This is the rule. You can choose get and you don't have to write

Content-Type entity header is used to indicate the MIME type media type of the resource.

in the response, the Content-Type header tells the client the content type of the actual content returned. Browsers do MIME lookups in some cases and do not necessarily follow the value of this title; to prevent this behavior, you can set the title X-Content-Type-Options to nosniff.

in a request, such as a POST or PUT), client, tells the server what type of data is actually sent.

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