What is the meaning of "accession" in the definition of "accessorial"?

[Tae-Bbong-E]

I have read an article containing the adjective "accessorial".
I've looked it up in Marrian webster dictionary.

Definition of ACCESSORIAL

of or relating to an accessory; of, relating to, or constituting an accession : supplementary… See the full definition

www.merriam-webster.com

It says two sorts of definitions but the thing is the second one.
2. : of, relating to, or constituting an accession: SUPPLEMENTARY
e.g.)
accessorial services

What is the meaning of accession in the second definition?

 

[5jj]

If you click on the word accession you are led to the definition.

 

[jutfrank]

Are you asking what 'accessorial services' are?

 

[Tae-Bbong-E]

I have clicked the word accession.
So regarding the number 2 definition of "accessorial", does the word "accession" refer to its number 3 definition?

● accessorial
2. : of, relating to, or constituting an accession: SUPPLEMENTARY
e.g.)
accessorial services


● accession

Definition of ACCESSION

the act or process by which someone rises to a position of honor or power; an act of coming near or to something : approach, admittance; something added : acquisition… See the full definition

www.merriam-webster.com