BLE Characteristics Permission

I was going through the tutorialCreate a Custom GATT Profile Characteristic on DA14531 and DA14585/DA14586 devices。While adding the new characteristic declaration, value and description in the custom server’s database attributes, we write:

// Control Point Characteristic Declaration [SVC1_IDX_CONTROL_POINT_CHAR] = {(uint8_t*)&att_decl_char, ATT_UUID_16_LEN, PERM(RD, ENABLE), 0, 0, NULL}, // Control Point Characteristic Value [SVC1_IDX_CONTROL_POINT_VAL] = {SVC1_CTRL_POINT_UUID_128, ATT_UUID_128_LEN, PERM(WR, ENABLE) | PERM(WRITE_REQ, ENABLE), DEF_SVC1_CTRL_POINT_CHAR_LEN, 0, NULL}, // Control Point Characteristic User Description [SVC1_IDX_CONTROL_POINT_USER_DESC] = {(uint8_t*)&att_desc_user_desc, ATT_UUID_16_LEN, PERM(RD, ENABLE), sizeof(DEF_SVC1_CONTROL_POINT_USER_DESC) - 1, sizeof(DEF_SVC1_CONTROL_POINT_USER_DESC) - 1, (uint8_t *) DEF_SVC1_CONTROL_POINT_USER_DESC},

I understand that PERM (RD, ENABLE) makes the declaration readable. But what does PERM(WR, ENABLE) | PERM(WRITE_REQ, ENABLE) imply? Is there a document containing all possible parameters for PERM and what each of them means?

Moreover, what does the 0 and NULL specify in the characteristic declaration. From this snippet, I understand that characteristic declaration, value and description conform to a particular format. What is this format exactly?