What does "Low density device" mean? About STM8 / STM32
They're arbitrary terms used by ST to distinguish different lines of MCUs within a series. Generally speaking, all of the parts within a line will have all the same peripherals and features; they will differ only in the amount of RAM and flash memory they have, and in packaging.

For instance, here's how ST breaks down the STM32F103 family:



The terms "value line" and "connectivity line" also appear in some series; those are also treated as lines.