It is considered plagiarism/copying when you "imitate or directly copy other people's texts, without specifying the source and pointing out that it is a matter of quotation or summary work" (Quote from Hanne Leth Andersen and Jens Toftskov: Exams and exam forms. Meaning and judgment. Samfundslitteratur 2008).
In an exam situation, plagiarism/copying means that you use a text, illustration, structure, idea, etc. in such a way that it appears as your own work.

If you reuse parts of or an entire assignment that you yourself have previously written, you must treat this assignment like any other source, i.e. you must cite your source correctly and mark the passage as a quotation. 
If you do not make references to your previous work, or otherwise explain that you are reusing your own material, you are misleading about the effort you have put into the performance at the exam in question.

If you get help from someone else to complete an exam that you have to take on your own, that is an examination irregularity.
In the case of written on-site examinations, any form of contact with fellow students or persons outside the examination room is considered an examination irregularity, irregardless of whether the contact concerns the exam or not.

If the course description or another source of information regarding the exam says that you are not allowed to bring aids, you must not bring anything to the exam. This applies, for example, to a collection of formulas, a dictionary, small "notes" or "post-its". Nor are you permitted to write small "nuggets of information" on your arm or elsewhere.
It will always be stated in the course-, or module description which rules apply for the individual exam.

If you collect empirical data, it will be falsification if you "invent" data. For example, if you need 100 answers and you have only managed to collect 90 answers, you are not allowed to invent the last 10 answers yourself.

If you have knowledge of the content of the examination assignment prior to the exam, and you still participate in the exam, this constitutes an examination irregularity.

For some teaching activities, attendance is mandatory, and attendance is considered part of the exam. If you get a fellow student to register you as in attendance even if you are not present, this is an examination irregularity. This is also the case if you enter a code as verification for presence without being present.
If you "help" a fellow student by registering them as present or share a code that is used for attendance verification, you are helping the fellow student cheat on the exam. Such actions are also considered examination irregularities and you could be sanctioned in the same way as if you had registered your own presence without being there.

Attempts to circumvent, disable, or otherwise impede the purpose of the University's use of electronic monitoring programs, including failure to connect to Exam Monitor, are also considered examination irregularities.