L4_IPC_DOPE() understanding

Hello

Currently im studying IPC for L4/fiasco. I opened the exercise solution from dresden and try to study the coding and its flow. So far, i manage to understand but there is one part that im not certain which is the L4_IPC_DOPE(). As stated in the exercise solution, the L4_IPC_DOPE is initiated as L4_IPC_DOPE(0,1) which is assigned to the l4_msgdope_t structure:-

typedef struct { l4_msgdope_t size_dope; }message_t; <---there are other variables initialize in this structure but i just shorten it to the issue.

message_t m;

m->size_dope = L4_IPC_DOPE(0,1);

I would like to know, what does this call (L4_IPC_DOPE) do and which file does this function is located. Is it in types.h? Hope anyone can assist me since im new in this field. Thanks.