PR101 refers to the synthetic earths, while PR102 is for naturally occurring red earths.
Red earths are useful in all manner of mixtures- to complement blues, to mix flesh tones, to create deep dark passages at low lightnesses or to create glowing chroma. Along with other earths (see yellow earth and brown earth) these earth tones have played a central role in art history and human cultural production.
The names for red earths often derive from places where certain red earths were historically mined. Each place has a distinctive set of qualities which helped to form the traditional names. "Venetian Red" or "Pozzouli Earth" harken to places where these colors were found. A cannon has arisen around these names such that synthetic colors with similar properties may borrow from the geographic names, but unfortunately this is not a guarantee as there is no standardization. These names are discussed below and may apply to synthetic or natural earths.
PR101 - Synthetic Iron Oxide Red. Also known as Red Oxide or Synthetic Red Ochre. This pigment group includes a huge variety of synthetically made red earths, oranges and violet-browns. Particle size can vary considerably and with the synthetic version it can sometimes be very small- though this is by no means a guarantee. Some brands, like Williamsburg, publish guides to the grainyness of their colors, while other brands rely on the brand vibe to convey the degree of homogenization among their colors (for example, some brands are known to make very smooth colors overall).
The Artist's Guide to Health and Safety has more information about health issues associated with pigments which contain Iron. Her updated work can be found through her site.
The PR101s are a wide-ranging bunch, and they can be red, brown, orange, violet or even greenish. They are super lightfast.
