4G/LTE - PHY Channel
Determining a Resource Allocation type is very tricky since many other parameters get involved. One informative table would help you with this job. As you see here, which Resource Allocation type to be used is determined by several factors like TM, No of CW, DCI format etc. However, you (eNB) may have a couple of choices even with the same condition (i.e, same TM, CW and DCI format) and it is all up to eNB radio stack implementation. For example, if eNB is configured to bet TM1, No of CW = 1 and use DCI format 1, it can use either type 0 or Type 1. Then, which type it will choose ? It is up to eNB decision making algorithm. Once eNB decided to which type it use, it specify the resource allocation type in DCI. If the eNB is set to be TM1, No of CW = 1 and decided to use DCI format 1A, it can only use Type 2.
As another example, if eNB is configured to be TM3 and CW = 2 (MIMO), it must use DCI format 2A, but it has choice of using Type 0 or Type 1. It is also up to eNB decision making algorithm to decide whether it will use type 0 or type 1.