1) Absolutely no relation to the series other than stylistically: An example is Clock Tower: Ghost Head. It retains the style of its predecessors, Clock Tower and Clock Tower 2. However, while Clock Tower and Clock Tower 2 feature recurring characters, villains, a storyline that flows together, Clock Tower: Ghost Head is an entirely new and different unrelated story with new characters, settings, etc.
2) Significant relation to the series but is not thought as part of the series: An example is Resident Evil Gaiden. Although it features characters and information from past games, it does not flow into the continuation of the series, and it just sort of happens (there are no events from other games that lead up to it, really).