https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Set-Cookie
Domain=<domain-value> Optional
Specifies those hosts to which the cookie will be sent.
If not specified, defaults to the host portion of the current document location (but not including subdomains).
Contrary to earlier specifications, leading dots in domain names are ignored.
If a domain is specified, subdomains are always included.
Path=<path-value> Optional
Indicates a URL path that must exist in the requested resource before sending the Cookie header.
The %x2F ("/") character is interpreted as a directory separator and sub directories will be matched as well (e.g. path=/docs, "/docs", "/docs/Web/", or "/docs/Web/HTTP" will all be matched).
https://developer.mozilla.org/en-US/docs/Web/HTTP/Cookies
Scope of cookies
The Domain and Path directives define the scope of the cookie: what URLs the cookies should be sent to.
Domain specifies allowed hosts to receive the cookie.
If unspecified, it defaults to the host of the current document location, excluding subdomains.
If Domain is specified, then subdomains are always included.
For example, if Domain=mozilla.org is set, then cookies are included on subdomains like developer.mozilla.org.
Path indicates a URL path that must exist in the requested URL in order to send the Cookie header. The %x2F ("/") character is considered a directory separator, and subdirectories will match as well.
For example, if Path=/docs is set, these paths will match:
/docs/docs/Web//docs/Web/HTTP
subdomain
https://en.wikipedia.org/wiki/Subdomain
A subdomain is a domain that is part of a larger domain; the only domain that is not also a subdomain is the root domain.[1]
For example, west.example.com and east.example.com are subdomains of the example.com domain, which in turn is a subdomain of the com top-level domain (TLD).
A "subdomain" expresses relative dependence, not absolute dependence: for example, wikipedia.org comprises a subdomain of the org domain, and en.wikipedia.org comprises a subdomain of the domain wikipedia.org.
https://en.wikipedia.org/wiki/HTTP_cookie
Cookie attributes
In addition to a name and value, cookies can also have one or more attributes.
Browsers do not include cookie attributes in requests to the server—they only send the cookie's name and value.
Cookie attributes are used by browsers to determine when to delete a cookie, block a cookie or whether to send a cookie to the server.
Domain and path
The Domain and Path attributes define the scope of the cookie. They essentially tell the browser what website the cookie belongs to. For obvious security reasons, cookies can only be set on the current resource's top domain and its sub domains, and not for another domain and its sub domains.
For example, the website example.org cannot set a cookie that has a domain of foo.com because this would allow the example.org website to control the cookies of foo.com.
If a cookie's Domain and Path attributes are not specified by the server, they default to the domain and path of the resource that was requested.[36] However, in most browsers there is a difference between a cookie set from foo.com without a domain, and a cookie set with the foo.com domain. In the former case, the cookie will only be sent for requests to foo.com, also known as a host-only cookie. In the latter case, all sub domains are also included (for example, docs.foo.com).[37][38] A notable exception to this general rule is Internet Explorer, which always sends cookies to sub domains regardless of whether the cookie was set with or without a domain.[39]
Below is an example of some Set-Cookie HTTP response headers that are sent from a website after a user logged in. The HTTP request was sent to a webpage within the docs.foo.com subdomain:
HTTP/1.0 200 OK
Set-Cookie: LSID=DQAAAK…Eaem_vYg; Path=/accounts; Expires=Wed, 13 Jan 2021 22:23:01 GMT; Secure; HttpOnly
Set-Cookie: HSID=AYQEVn…DKrdst; Domain=.foo.com; Path=/; Expires=Wed, 13 Jan 2021 22:23:01 GMT; HttpOnly
Set-Cookie: SSID=Ap4P…GTEq; Domain=foo.com; Path=/; Expires=Wed, 13 Jan 2021 22:23:01 GMT; Secure; HttpOnly
…
The first cookie, LSID, has no Domain attribute, and has a Path attribute set to /accounts. This tells the browser to use the cookie only when requesting pages contained in docs.foo.com/accounts (the domain is derived from the request domain).
The other two cookies, HSID and SSID, would be used when the browser requests any subdomain in .foo.com on any path (for example www.foo.com/bar). The prepending dot is optional in recent standards, but can be added for compatibility with RFC 2109 based implementations.[40]