Will other WEEE systems or companies see my data when I use the sector of the WF-RepTool server?

No, only you as WEEE system / company who uses the WF-RepTool server sector (see Where can the WF-RepTool be installed?) will see your own data, data of your treatment partners.
Also the WEEE Forum has no access to your data.
The WF-RepTool IT manager may look into your data to check if e.g. the installation of any new WF-RepTool version was successful. He works under a clear confidentiality agreement.