External activities constitute, e.g., student exchange visits, prolonged professional activities outside the usual workplace, or international exchange semesters.
In this case standard deductions for the trip (0,30 EUR per kilometre) and for the additional meal allowance apply, without the need for receipts.
In case the absence due to external activity exceeds 8 hours, you are entitled to 12 EUR per day. If it exceeds 24 hours, the sum doubles to 24 EUR (as of 2016).
Only full days and no partial days can be counted as absences.
The trips and the additional meal allowances are independent of one another.
Keep in mind that you cannot deduct any such expenses if they were reimbursed by your employer. If the costs were only partially reimbursed, you can still deduct your share.