Mike,

It's (originally) a US term, meaning fragment of a network. I believe it was first used in the 1960s or thereabouts, in the context of using hand-drawn networks to present extension of time claims (this was after CPM had been invented but before PCs). It is cumbersome to reproduce a network fully by hand, so only the relevant fragment of it would be reproduced in presenting EOT claims. Change could be mapped by hand by superimposing a fragnet on a network using tracing paper, or using a red pen, or something like that. The term 'fragnet' I believe entered the public domain when an analysis technique employing fragnets was cited with approval in a US Federal Board of Contract Appeals case (don't ask me which one) in the late 1960s or early 1970s. With that judicial approval the term became more widely adopted.

I wouldn't read too much into Primavera's use of the term, from the 1980s onwards. The Primavera developers, I imagine, decided to add a new feature so that users could save only a part or section of a project. They would have used the term "fragnet", I guess, because it was a sexier sounding buzzword than "part" or "section".