ASBR summary question about generating blackhole route

[tghgrs]

I'm currently implementing the function to generate blackhole route during ASBR summarization in OSPF. I'm referring to the two functions ospf_add_discard_route() and ospf_delete_discard_route().I don't quite understand why we need to differentiate between NSSA areas and non-NSSA areas.
For this code segment:
if (!nssa && or->path_type == OSPF_PATH_INTRA_AREA) { if (IS_DEBUG_OSPF_EVENT) zlog_debug("%s: an intra-area route exists", __func__); return 0; }
Why doesn't it include NSSA areas?
And for this part:
if (nssa) new_or->path_type = OSPF_PATH_TYPE2_EXTERNAL; else new_or->path_type = OSPF_PATH_INTER_AREA;
Why is the path_type chosen this way?"