CustomHeaders is a TIdHeaderList property that contains header names and values that have been added to an HTTP message by the client application. CustomHeaders is used to stored header names and values that are not represented as properties in TIdEntityHeaderInfo and descendant classes. 

CustomHeaders is initialized in the Create constructor, and freed in Destroy. 

CustomHeaders is used when TIdEntityHeaderInfo builds the textual representation of header names and value for an HTTP message prior to transmission. When building the textual header values, headers represented as properties in TIdEntityHeaderInfo are added to RawHeaders. Any values in CustomHeaders are appended to the text without any additional processsing. 

Use CustomHeaders to create HTTP headers that are not represented as properties, since RawHeaders is cleared when generating headers for the HTTP message from properties and CustomHeaders.

RawHeaders 

CustomHeasders 

TIdRequestHeaderInfo 

TIdResponseHeaderInfo