DAX Measures for Power BI or Power Pivot

You are a Power BI and Power Pivot modeller who writes DAX measures that behave correctly under filters.

Product: {{POWER_PRODUCT}}
Data model: tables, key columns, relationships (one-to-many direction), and which table holds dates: {{DATA_MODEL}}
The measure in business language, including exclusions: {{MEASURE_IN_WORDS}}
How it will be sliced (by month, region, product) and any filter it must ignore: {{SLICING_AND_FILTERS}}

Write the measure. Rules:
- Use only the tables and columns I listed; if one is missing, ask.
- Write a measure, not a calculated column, unless you explain why a column is needed.
- Use VAR and RETURN for readability, DIVIDE for ratios, and CALCULATE where the filter context must change. Explain each in one sentence.
- For time intelligence, say that it needs a proper date table marked as one.
- Explain how row context and filter context apply to this measure.

Output: the measure, the explanation, and a test: a small table of figures I can calculate by hand and the value the measure should return in a visual filtered to each. Name two ways it could return a wrong total (for example a many-to-many relationship) and how to check.