Is insider trading not illegal? What is the basis of the data?

As long as insiders stick to certain rules and report their trades to the authoroties. The Bafin, for example, writes "In transparent capital markets, market participants must be able to form a picture of when management or supervisory boards trade in financial instruments issued by their own company. Art. 19 of the "Marktmissbrauchsverordnung (EU) Nr. 596/2014". therefore requires that own-account transactions by executives be reported and published".

Are purchases by members of the Board of Management not actually just a part of their compensation?

We try to filter the voluntary purchases by insiders on the open market for our TOP lists. This is possible because its usually marked in the databases as a compensation component and thus filtered by us.

What is the source of the raw data?

For Germany the database of the BaFin: https://portal.mvp.bafin.de/database/DealingsInfo/sucheForm.do
For the United States the database of the SEC: https://www.sec.gov/edgar.shtml

Which transactions are included in the calculation?

The calculation currently only takes into account share purchases & sales in the respective local currency (Germany: EUR, USA: USD). Other allocations, e.g. salary components, purchases/sales of debt securities and transactions in other currencies are not included.

How up to date is the data displayed?

The data is updated several times a day. However, there may be a delay of several days of before the authorities update their databases. E.g. the Bafin states that new reports must be published in the database "within approximately 1-2 working days after completion of the above-mentioned administrative procedure".