Describe/define your (possible) variables of interest in your proposal. (My topic is about the ongoing labor shortage that is affecting the United States) [see the attached paper you wrote]
You can include conceptual definitions as well as operational definitions.
Please refer to the lecture of variables before the discussion.
https://support.minitab.com/en-us/minitab-express/1/help-and-how-to/modeling-statistics/regression/supporting-topics/basics/what-are-categorical-discrete-and-continuous-variables/
Understanding the Differences Between Constructs, Variables, and Operational Definitions
https://conjointly.com/kb/understanding-variables/
This requires no sources.