Why A formal ARRL radiogram preamble contains the message number, precedence, handling instructions, station of origin, check, place of origin, time filed and date. The "check" is the count of words or word equivalents in the text only, not the address or signature. The receiving operator counts the text as it is copied and compares it to the check; if the numbers disagree, something was lost or added in relay and the message must be corrected before passing it on.
Watch out The station of origin is its own separate field in the preamble, so the choice naming the originating station's call sign describes a different part of the header.
Check = the count. Like balancing a checkbook: count the text words and see if the numbers agree.