The word "内详" is formed by combining the character "内" (nèi), meaning "inside" or "internal", with "详" (xiáng), meaning "details" or "particulars". Together, they create a directive phrase used primarily on envelopes to indicate that the sender's information is contained within the correspondence, literally conveying "details inside". The formation is semantic, with "内" specifying the location where the "详" can be found.