Generally, when you create a noun from a (transitive) verb X with -ung, the resulting noun describes the act of doing X or the process of x.
- umarmen - Umarmung (the act of embracing)
- fertigen - Fertigung (the process of producing something)
This applies to the majority of cases, yet there are some exceptions with overlap/variation, i.e.:
- Pflanzung (from pflanzen -> to plant) could technically mean the act of planting but would more likely refer to the resulting plantation. In that case the act of planting would rather be described as das Pflanzen.
„Außerhalb der Stadt gibt es eine neue Pflanzung von Weihnachtsbäumen. Das Pflanzen der Bäume dauerte fast einen Monat.“
- Besitzung (often used in plural) always means "property" (with a focus on (farm) land or real estate). The act of owning will always be das Besitzen.
Also usually intransitive and intransitively used transitive verbs will not be nominalised with -ung, but rather by capitalising the infinitive (essen, schlafen, spielen, malen, lieben...).
In the case of Einschreiben/Einschreibung, both stem from the root of the process of writing something into something, with the first being a piece of mail that requires the recipient to sign in with the postman in order to confirm the delivery (giving written confirmation), while the other stems from the process of writing your name into a list of (potential) participants.
It should noted though that das Einschreiben in reference to mail is more of a "product name" with overloaded meaning. You can technically still use the infinitive based nominalisation as a process description (writing things into something), though it may sound anachronistic/wrong as you would nowadays rather use Eintragen to achieve that meaning.