Sectors.
People may all have the same basic personality traits, and the key structures of most organisations are also similar. But the interaction between the two, and the employment culture that is created tends to develop in line with the industry sector.
Good HR strategy and practice is completely integrated with the organisation's aims and therefore whilst the generic HR skills are transferable, experience of the organisation type can be critical to its successful implementation.
A breadth of sector knowledge and organisation type enables cross-fertilisation of ideas, supporting innovation and avoiding a "me too" approach.