> For the complete documentation index, see [llms.txt](https://docs.lucernahealth.com/guide/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.lucernahealth.com/guide/insights-studio/population-builder/reference-guides/what-is-a-block.md).
# What is a Block
A block is a reusable set of filter conditions that can be referenced inside populations and other blocks.
Blocks don't run on their own. They produce no member lists and no saved counts. A block takes effect only as part of whatever references it.
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### Why blocks exist
Logic like "active roster member" appears across many populations. Defining it once as a block means everyone references the same definition, and a correction made in one place carries everywhere it's used.
Without blocks, the same logic gets rebuilt each time it's needed. Small differences creep in, for example one person filters on a status field, another on an eligibility date , and as a result two populations that should match don't.
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### Blocks and populations
Both are built the same way, using the same query builder. The difference is what they produce and how they're used.
| | Block | Population |
| ---------------------------- | ----- | ---------- |
| Runs on its own | No | Yes |
| Produces a member list | No | Yes |
| Can be scheduled | No | Yes |
| Can be sent to a destination | No | Yes |
| Can be referenced by others | Yes | Yes |
| Can reference a population | No | Yes |
A block cannot reference a population. When you build a block, the query builder offers Add Block and Add Field only.
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### When to use which
Use a **block** when the logic is general enough that other people will need it. "Active roster member," "Florida members," "adults 19–63" — these are building blocks other queries sit on top of.
Use a **population** when you have a complete list you'd act on directly. "Diabetic members with an open eye exam" is something you'd send to a destination, not something you'd filter by.
Note: if you find yourself rebuilding the same conditions across several populations, that logic belongs in a block. If a set of conditions only makes sense inside one population, leave it as rules in that population.
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### Finding blocks
Use the search bar to find a block by name, or use the filters to narrow the list. Filters are available for Category and Type, with more under Add Filter.
Filters can be saved for repeated use. Click All Views to find a saved view, then click Apply. To save one, apply your filters, click Save View, complete the details, and click Save. Views are public by default, toggle off Public to keep a view private.
Blocks tagged **System** are created and managed by the Lucerna team. You can view the full definition and logic, but not edit it. Blocks tagged **Catalog** ship with Population Builder and are pushed out to all clients.
Search the Blocks tab before building anything new. Reusing an existing block is faster than rebuilding it, and it avoids introducing a second definition of something that already exists.
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### Where a block is used
The References tab on a block shows which populations and other blocks reference it.
Check this before editing. Any change you make carries through to everything referencing the block, so an edit that looks small can affect queries well beyond the one you're working on.
