Explanation
Connection to Autonomous DB uses certificate authentication and Secure Sockets Layer (SSL). This ensures that there is no unauthorized access to Autonomous DB and that communications between the client and server are fully encrypted and cannot be intercepted or altered.
Certification authentication uses an encrypted key stored in a wallet on both the client (where the application is running) and the server (where your database service on the Autonomous DB is running). The key on the client must match the key on the server to make a connection There's credentials zip file available for download once ADB is created, the zip file contains a collection of files, that needed to connect to your Autonomous DB. All communications between the client and the server are encrypted Wallet files, along with the Database user ID and password provide access to data in your Autonomous Transaction Processing database. Store wallet files in a secure location. Share wallet files only with authorized users. If wallet files are transmitted in a way that might be accessed by unauthorized users (for example, over public email), transmit the wallet password separately and securely.
For better security, Oracle recommends using restricted permissions on wallet files. This means setting the file permissions to 600 on Linux/Unix.
Similar restrictions can be achieved on Windows by letting the file owner have Read and Write permissions while all other users have no permissions.
https://docs.cloud.oracle.com/en-us/iaas/Content/Database/Tasks/adbconnecting.htm
https://docs.oracle.com/en/cloud/paas/autonomous-data-warehouse-cloud/user/connect-download-wallet.html#G

Explanation
The tnsnames.ora file provided with the credentials zip file contains five database service names identifiable as tpurgent, tp, high, medium, and low. The predefined service names provide different levels of performance and concurrency for Autonomous Transaction Processing.
tpurgent: The highest priority application connection service for time critical transaction processing operations. This connection service supports manual parallelism.
tp: A typical application connection service for transaction processing operations. This connection service does not run with parallelism.
high: A high priority application connection service for reporting and batch operations. All operations run in parallel and are subject to queuing.
medium: A typical application connection service for reporting and batch operations. All operations run in parallel and are subject to queuing. Using this service the degree of parallelism is limited to four (4).
low: A lowest priority application connection service for reporting or batch processing operations. This connection service does not run with parallelism.