For a Standard account, you can only sign up for free topics.
For a PREMIUM account, you can sign up for all the topics that are still valid.
Steps to register a topic (when an account is allowed) as follows:
Step 1: To subscribe to a topic, you need to visit Extended topics page.
Step 2: You can click the Register button to participate in that Topic. Then press the Access button. If the topic has expired, you can no longer register.