Thesis material consists of all of the data that has been collected or otherwise acquired to be used in the thesis. This material can consist of survey results, laboratory tests and other measurements or surveys. The materials can also be collected from research & development publications or from open access publications.
It's good to have a data management plan that tells how all of the collected materials will be stored or if it's going to be destroyed. When and how that happens, if it happens. Or is the material something that can also be published in an open access archive. It is also good to have an agreement on who gets to use the materials after the thesis is done. For example, if the thesis had multiple persons writing it, some of them might want to use the collected materials as a basis for later studies.
Guidance and template for the data management plan is on Arene's website. Read the instructions first and take out the first page of template, then fill the form as guided. Data management plan can be saved in WIHI. The guidance is below the Ethical instructions and the address to Arene's page is https://arene.fi/julkaisut/raportit/opinnaytetoiden-eettiset-suositukset/.