Frames of reference. Both mean files not fully downloaded. Partial file sharing was a term used by LW 4 referring to a file you were downloading, portions of which were shared back to others.

Incomplete simply means the file is partially downloaded but not complete. With LW 4 was the incomplete folder as a separate listing in the Library window. People sometimes mistakenly shared this folder. This was bad for everyone. Instead best option was Partial File Sharing, which could be set under Sharing -> advanced -> partial files option.

LW 5 users do not need to even think about this option now, it's made easy.