You can configure a dimension so that information for other dimensions is automatically entered when you enter that dimension in a document. For example, if you enter cost center 10, a value of 20 can be automatically entered in the department dimension. You can also set up the derived dimensions segments and values by using entities.

By default, the derived dimension process doesn't override existing values for derived dimensions. For example, if you enter cost center 10, and no other dimension is entered, department 20 and location 30 are entered by default. However, if you change the cost center, the values that have already been established aren't changed. Therefore, you can establish default dimensions on master records, and those dimensions won't be changed by derived dimensions.

In 8.1.2, you can change the behavior of derived dimensions to override existing values by selecting the Replace existing dimension values with derived values check box on the Derived dimensions page. Derived dimensions with this setting do not automatically replace the existing default dimensions values when dimension values are defaulted. Dimension values will only be overridden when you enter a new dimension value on a page and there are existing derived values for that dimension on the page.

When you use Add segment on the Derived dimensions page to add a segment as a derived dimension, an option is provided at the bottom of the Add segment page that allows you to prevent changes to that dimension when it is derived on a page. The default setting is off so it does not prevent the derived dimension values from being changed. Change the setting to Yes if you want prevent the dimension from being changed after it has been derived. You can also change the value using Segment Actions 

For more information, follow this link:

https://docs.microsoft.com/en-us/dynamics365/unified-operations/financials/general-ledger/financial-dimensions 

You can configure a dimension so that information for other dimensions is automatically entered when you enter that dimension in a document. For example, if you enter cost center 10, a value of 20 can be automatically entered in the department dimension.