I have tried to look up the rules of the Democratic primary process. It seems that states have two sets of statewide pledged delegates: pledged at-large delegates and pledged PLEO delegates. What is the difference between those two?
The acronym PLEO stands for "Party Leader and Elected Official". Generally speaking, there are three types of delegates sent to the convention: normal delegates, who are chosen at the district level; at-large delegates, who are chosen state-wide; and PLEO delegates, who are like at-large delegates except they are hand-picked from the party leadership. The idea, ostensibly, is to give representation at the convention to local districts, the state as a a whole, and the party itself.
These delegates are all pledeged, meaning that they are expected to represent the choices and interests of the group that chose them. There are also automatic delegates (also called superdelegates) who are high muckety-mucks in the party: ranking DNC members, current or ex presidents, Democratic members of congress and Democratic governors, etc. These people are not pledged to support any particular candidate, but are free to use their best discretion. See: Democratic delegate rules, 2020