The LOOKUP function is used to return the value of a specified field in a different row, which can be defined relative to the current row. In this context, it can be used to include a number with the position of each state within each region by looking up the SUM([Quantity]) in each respective position. References: The information is inferred from general Tableau knowledge as well as the following sources:
Tableau Functions (Alphabetical)
LOOKUP
Tableau Certified Data Analyst Exam Guide

Explanation:
The object that you should add to the dashboard to allow users to switch to a different dashboard in the workbook is Navigation.
Navigation objects are used to create links between dashboards or sheets in a workbook. You can add a navigation object to a dashboard by dragging it from the list of objects and dropping it on the dashboard. You can then configure the navigation object to specify the destination dashboard or sheet that you want users to navigate to when they click on it.
You can learn more about navigation objects and how to use them in this article:
https://help.tableau.com/current/pro/desktop/en-us/dashboards_navigation.htm

check the steps below in explanation
Explanation:
To modify the bar chart to show the percentage of sales for each Subcategory within every Category, you need to do the following steps:
Open the link to Book1 found on the desktop. This will open the Tableau workbook that contains the CategoryPercentage worksheet.
Click on the CategoryPercentage tab at the bottom of the workbook to open the worksheet. You will see a bar chart that shows the total sales for each Subcategory across all Categories.
Right-click on Sales on the Columns shelf and select Add Table Calculation from the menu. This will open a dialog box that allows you to apply a calculation to your measure.
Select Percent of Total from the Calculation Type drop-down list. This will calculate the percentage of sales for each Subcategory out of the total sales for all Subcategories.
Select Category from the Compute Using drop-down list. This will calculate the percentage of sales for each Subcategory within every Category, instead of across all Categories. The total percentage for every Category will be 100%.
Click OK to apply the table calculation. You will see that the bar chart now shows the percentage of sales for each Subcategory within every Category.