CRM uses tracking token to identify email conversations by adding alphanumeric strings to the email subject line.
This token looks like CRM:0111222. This token contains CRM:0111222.
Configurable from 1-20 chars. Can be unique for each organisation or Dynamics 365 instance.
Configurable from 0-2,147,483,647. Can be used as an identifier for a specific instance, organization or deployment of Dynamics 365.
Configurable from 1-9. Determines how many digits to use when Microsoft Dynamics 365 generates the numeric identifier for the Microsoft Dynamics 365 user who generated the email activity.
The number of digits is determined by the number of users you have (important to avoid duplicate). Eg. 1 digit handles up to 9 users. 2 digits handle up to 99 users, and so on.
Configurable from 1-9. Determines how many digits to use when Microsoft Dynamics 365 generates the numeric identifier for the email activity (not the individual messages that the activity contains).
Similar concept to user number, however it is less certain with the number of email conversations. Eg. 1 digit handles up to 9 emails for each user. 2 digits handle up to 99 emails for each users, and so on.
For more details, read technet article here.
Original article by Aaron Richards, here.