How do I use a main form field inside a child table?

Short answer: Table formulas can see only fields in the same table. Add a computed column with Parent.Field_ID to copy the main form value into every row.

Why it happens

A table column's formula works on its own row. Fields in a section have to be brought into the table through the Parent function. Table-to-table references aren't supported.

How to fix it

1. In the table, add a column and make it a computed field.
2. Enter Parent.Field_ID. See the Parent function.
3. Use that column in other table formulas.

For a Yes/No field outside the table, convert it to text: if(Parent.YesNo_field = true(), "Yes", "No").

To show or hide a column based on a section field:

1. Select the column and open its visibility settings.
2. Add a condition on the section field only. The rule then applies to the whole column across all rows.
3. If the condition also uses a table field, it applies row by row instead.