The InField property retrieves or specifies the "request" fields. If the FieldName parameter is empty,  the InField property retrieves all the fields in the GET header. Use the InField property to add addition fields to the GET request. The RFC 2616, explains better all the fields that may be used. The OutField property lists the repose fields. Use the GET method to retrieve a document or a web page.

Shortly here they are:

• Accept, The Accept request-header field can be used to specify certain media types which are acceptable for the response. By default, the Accept field is "*/*"
• Accept-Charset, The Accept-Charset request-header field can be used to indicate what character sets are acceptable for the response. By default, the Accept-Charset field is not used.
• Accept-Encoding, The Accept-Encoding request-header field is similar to Accept, but restricts the content-codings that are acceptable in the response. By default, the Accept-Encoding field is not used.
• Accept-Language, The Accept-Language request-header field is similar to Accept, but restricts the set of natural languages that are preferred as a response to the request. Language tags. By default, the Accept-Language field is "en-us".
• Authorization, A user agent that wishes to authenticate itself with a server-- usually, but not necessarily, after receiving a 401 response--does so by including an Authorization request-header field with the request. By default, this field is not used.
• Expect, The Expect request-header field is used to indicate that particular server behaviors are required by the client, By default, this field is not used. 
• From, The From request-header field, if given, SHOULD contain an Internet e-mail address for the human user who controls the requesting user agent. By default, this field is not used
• Host. The Host request-header field specifies the Internet host and port number of the resource being requested, as obtained from the original URI given by the user or referring resource (generally an HTTP URL). This field is automatically get from the URL.
• If-Match, The If-Match request-header field is used with a method to make it conditional. A client that has one or more entities previously obtained from the resource can verify that one of those entities is current by including a list of their associated entity tags in the If-Match header field. By default, this field is not used.
• If-Modified-Since, The If-Modified-Since request-header field is used with a method to make it conditional: if the requested variant has not been modified since the time specified in this field, an entity will not be returned from the server; instead, a 304 (not modified) response will be returned without any message-body. This field is not used.
• If-None-Match, The If-None-Match request-header field is used with a method to make it conditional. A client that has one or more entities previously obtained from the resource can verify that none of those entities is current by including a list of their associated entity tags in the If-None-Match header field. By default, this field is not used.
• If-Range, If a client has a partial copy of an entity in its cache, and wishes to have an up-to-date copy of the entire entity in its cache, it could use the Range request-header with a conditional GET (using either or both of If-Unmodified-Since and If-Match.) However, if the condition fails because the entity has been modified, the client would then have to make a second request to obtain the entire current entity-body. By default, this field is not used.
• If-Unmodified-Since, The If-Unmodified-Since request-header field is used with a method to make it conditional. If the requested resource has not been modified since the time specified in this field, the server SHOULD perform the requested operation as if the If-Unmodified-Since header were not present. By default, this field is not used.
• Proxy-Authorization, The Proxy-Authorization request-header field allows the client to identify itself (or its user) to a proxy which requires authentication. The Proxy-Authorization field value consists of credentials containing the authentication information of the user agent for the proxy and/or realm of the resource being requested. By default, this field is not used.
• Referer, The Referer[sic] request-header field allows the client to specify, for the server's benefit, the address (URI) of the resource from which the Request-URI was obtained (the "referrer", although the header field is misspelled.) The Referer request-header allows a server to generate lists of back-links to resources for interest, logging, optimized caching, etc. This field can not be changed.
• TE, The TE request-header field indicates what extension transfer-codings it is willing to accept in the response and whether or not it is willing to accept trailer fields in a chunked transfer-coding. By default, this field is not used. 
• User-Agent, The User-Agent request-header field contains information about the user agent originating the request. This is for statistical purposes, the tracing of protocol violations, and automated recognition of user agents for the sake of tailoring responses to avoid particular user agent limitations. By default, this field is "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.0)"
• Vary, The Vary field value indicates the set of request-header fields that fully determines, while the response is fresh, whether a cache is permitted to use the response to reply to a subsequent request without revalidation. By default, this field is not used.