BPOU stands for Basic Political Operating Unit. What that really means though, is that a BPOU is the lowest level of political organization. In rural areas, a BPOU may be an entire county. However, in urban areas, the BPOU encompasses only a particular Senate District or even a single House District.
The BPOU operates as a fairly independent group. It has it's own leadership, membership, budget, etc. Each BPOU endorses members during election years at it's caucuses. And during off-election years holds conventions to elect officers and other business.
However, a BPOU is also part of hierarchy. Unless, the BPOU is a House District, it will have House Districts within it. Each House District is representated in the state legislature by a Representative. In addition, each BPOU encompasses quite a few precincts. These precincts vary in size but are used to help organize votes and voting locations on Election Day.
A BPOU may contain an entire city, or only part of one. It may contain several cities, orpart of several. A BPOU may encompasses an entiry county, or only part of one. It may also contain parts of several counties as well.
In the case of SD37, our BPOU encompasses Senate District 37, which comprises House Districts 37A and 37B. While SD37 is entirely within Dakota County, it includes the cities of Apple Valley, Rosemount and portions of Burnsville. In particular, it includes the all 16 precincts of Apple Valley, all 7 precincts of Rosemount, and 4 precincts of Burnsville.
To make this even more confusing, the boundaries of a BPOU can change when the legislature does it's redistricting every ten years.