The Employment Permit is a document which TU Delft must obtain in order to employ staff of certain nationalities. Note that 'employment' is subject to a very broad definition: internships and PhD practical studies are also regarded as employment, even if unsalaried.
Moreover, there need not be an official appointment or contract of employment. The only consideration is whether activities are being conducted for or on behalf of someone else.
The purpose of the Employment Permit is to protect the Dutch labour market. Accordingly, it will not be issued if there is already someone in the Netherlands who can perform the intended work, and is available to do so. This is assessed by the (UWV), from whom TU Delft must obtain the Employment Permit.