In 3E Exchange, Products are composed of Materials. Each Material in turn can be composed of sub-materials.

Use Levels to represent the nesting relationship of Materials. Levels create a "hierarchy" within a Product.

Levels can be arbitrarily deep. For example, main materials on Level 1, sub-materials on Level 2, and substances on Level 3.
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Use Linked Materials to construct Products from Materials that are automatically updated when the Material is updated. Materials themselves may also contain Linked Materials.

For more information about creating and attaching these materials, see Linked Materials.
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When working with a Bill of Materials (BOM), 3E Exchange can automatically calculate values for weight-related columns. You should choose an auto-calculation mode based on what information you have available, and what information you'd like 3E Exchange to calculate.

For general information about auto-calculation options and choosing the right mode for your materials and products, see Weight Auto-calculations.

In the mode Weight (Absolute) the Weight of each row is calculated by multiplying the parent part's Weight and the row's Material % by Weight. 

In the mode Quantities/Unit Weights, the top-level Weight is calculated by multiplying the Unit Weight and Quantity (note: Quantity is defaulted to "1" if not provided). The Weight of each row is then calculated by multiplying the parent part's Weight and the row's Material % by Weight. 

In the mode Percentage (Relative), the Product % by Weight of each row is calculated by multiplying the parent part's Product % by Weight and the row's Material % by Weight.

In the modes Weight (Absolute) and Quantities/Unit Weights, Product % by Weight is determined by first finding a Total Weight for the product and then dividing the individual Weight of each row by the Total Weight.

There are two methods for determining a Total Weight:

When using Substance Rows for Summing Total Weights, 3E Exchange will add together the weights of all lowest-level substances in the product hierarchy to find the Total Weight.

When using Absolute Weights Entered for Summing Total Weights, 3E Exchange will find and sum the first Absolute Weight rows in the product hierarchy. An Absolute Weight is a row in which a Weight is provided without a Material % by Weight; this value is deemed to be the total weight of the row (and its children) within the product.

Here are some examples using the auto-calculation methods and linked materials.

In the example below, the product consists of two linked materials and Product % by Weight is auto-calculated using the product's Product % by Weight and linked materials' Material % by Weight.

In the example below, the product consists of two linked materials and the spreadsheet is using the mode Weights (Absolute) - Use Substance Rows for Summing Total Weights

First, weights for each row are calculated based on the parent row's Weight and the row's Material % by Weight. Second, Product % by Weight is calculated based on the sum of all substance-level row weights in the product.

In the example below, the product consists of one part containing two linked materials, and a second part containing a single substance. The spreadsheet is using the mode Quantities/Unit Weights - Use Substance Rows for Summing Total Weights
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First, the Weights of the top-level rows are calculated by multiplying the Quantity and Unit Weight. Second, weights for each row are calculated based on the parent row's Weight (calculated in the previous step) and the row's Material % by Weight. Second, Product % by Weight is calculated based on the sum of all substance-level row weights in the product.