Apex Processes on Contracts and Renewals in Salesforce CPQ

Contract in Foreground = False

Selecting the Contracted Checkbox from the Opportunity will generate the contract, subscriptions, and assets via a Future Apex Method.
Selecting the Contracted Checkbox on an Order will generate the contract, subscriptions, and assets via Queueable Apex.
Note: Contract record is created first. This means that if you attempt to automatically renew a contract upon creation when contracting, you must perform your renewal action via a time-delayed workflow rule or use a batch job. The subscription and/or asset records may not be available when the renewal process runs.

Use Legacy Amend/Renew Service = True

Selecting Renewal Forecast on a contract performs the creation of the Renewal Opportunity via synchronous Apex code execution. It will then queue a calculation to sync the information to the Opportunity Products.
Selecting Renewal Quoted on a contract performs the creation of the Renewal Quote and Quote Lines via synchronous Apex code execution. It will then queue a calculation to properly calculate the quote and update the Quote, Quote Lines, and Opportunity Products.
If a contract has Renewal Forecast set to true and you have yet to create a quote on the renewal opportunity, Salesforce CPQ will update the Opportunity Products on the renewal Opportunity with the data from the Quote Lines on the amendment Quote via queueable Apex when you contract an amendment Opportunity or amendment Order.
If there has been a quote created on the renewal Opportunity, then contracting an amendment will not create a queueable job to update the renewal Opportunity Products.

Use Legacy Amend/Renew Service = False

Selecting Renewal Forecast on a contract performs the creation of the Renewal Opportunity and its Opportunity Products via an asynchronous process and a third party service, Heroku. A queueable job is created to perform a callout to Heroku.
Selecting Renewal Quoted on a contract performs the creation of the Renewal Quote and Quote Lines via an asynchronous process and a third party service, Heroku. A queueable job is created to perform a callout to Heroku. There is no additional queueable job to perform a calculate. All records created are already calculated in Heroku.
If a contract has Renewal Forecast set to true and you have yet to create a quote on the renewal opportunity, a queueable job is created to perform a callout to Heroku.
If there has been a quote created on the renewal Opportunity, then contracting an amendment will not create a queueable job to update the renewal Opportunity Products.
When selecting multiple contracts to be renewed in an account, a contract selected as master cannot have an existing renewal opportunity.
Learn more about the Large Scale Amend/Renewal Service.