Generally, only those papers researched and written after enrolment can be included and only those actually accepted for publication are allowed (. not those submitted and awaiting acceptance).
Papers must be accepted by reputable, high profile journals which require full peer review of contributions.
You should be the main contributor to all of the papers you include (. you have been responsible for the key ideas, the development of the study and the writing of the paper). It is possible to include papers co-written with other authors as long as you have their permission (preferably in writing).
The papers submitted must form a coherent whole – they must be linked thematically (by a consistent focus on a particular topic) and structurally (by the inclusion of an introduction, explanatory material between the chapters and a conclusion). Normal academic requirements apply: your thesis must be a substantial, original contribution to knowledge in the discipline and must observe appropriate referencing conventions, etc.