sending vs transmittal

sending and transmittal both are nouns.

Word	Noun	Adjective	Verb	Adverb
sending	Yes	No	No	No
transmittal	Yes	No	No	No

As nouns, transmittal is a hyponym of sending; that is, transmittal is a word with a more specific, narrower meaning than sending:

sending: the act of causing something to go (especially messages)
transmittal: the act of sending a message; causing a message to be transmitted

Other hyponyms of sending include transmission, transmitting.

sending (noun)	transmittal (noun)
the act of causing something to go (especially messages)	the act of sending a message; causing a message to be transmitted