The Index of Sources is the collection of Late Middle English texts which were collected and examined for the creation of the atlas.
Many of those texts were not deemed linguistically interesting enough for further analysis, most often because their language was standardised, and did not include regionally varied forms. Consequently, there are no Linguistic Profiles for them, and their id starts with noLP_, and does not contain a link.
The Index of Sources entries which do have a Linguistic Profile have an id starting with lp_, and you can see that Linguistic Profile by clicking on their id link.
For a detailed description, see Introduction to the Index of Sources.

The corpus column registers the subset of those texts in the Index of Sources which do have a Liguistic Profile. There are various methodological differences between them.
north: Liguistic Profiles with id lp_0001-lp_2000. These linguistic profiles correspond to texts from counties north of the Wash (roughtly the line from Boston in the east to Stoke-on-Trent in the west). The collection of forms in this sub-corpus tends to comprise full reports for strictly delimited samples of the manuscript, but those manuscripts are seldom checked beyond the analysed sample.
south: Liguistic Profiles with id lp_5000-lp_9999. These linguistic profiles correspond to texts from counties south of the Wash. The collection of forms in this sub-corpus tends to comprise full reports for strictly delimited samples of the manuscript, but these forms are supplemented by scanning the rest of the manuscript for items not found in the initial sample.
north-south: Liguistic Profiles with id lp_4000-lp_4999. They are 60 profiles from the Midlands, Norforlk and Suffolk. They were investigated as part of both the North and the South corpus, and their Linguistic Profiles are the conflated results of the two surveys.

You will notice each row in the Index of Sources may contain only a subset of the different fields in the contents column.