GitHub, GitLab, Jira, and Trello: It is the datetime of the first user assigned event, which happens when a user is added to the list of assignees for the first time.

ClickUp: Since this app does not expose historical events, Keypup tracks changes on the assignees field to detect this timestamp. This tracking only applies to assignments performed after the project has been connected to Keypup. This field remains null when we detect that reviewers were assigned before importing the issue or pull request to Keypup because we cannot know precisely when this assignment occurred.