Explanatory Sequential Design - Two phase design where quantitative data is collected and analyzed first, then qualitative data is collected and analyzed based on the quantitative results. The qualitative data is used to explain the quantitative data.
Exploratory Sequential Design - Three phase design where qualitative data is collected and analyzed to inform theory, instrument, etc. for testing. The quantitative phase is used to test the theory, instrument, etc. with the results further developing the theory, instrument, etc.
Creswell, J.W., & Creswell, J.D. (2018). Mixed methods procedures. In, Research design: Qualitative, quantitative, and mixed methods approaches (5th ed., pp. 213-246). Los Angeles, CA: SAGE Publications, Inc.