When setting tables in an app form, keep in mind that tables have the following limitations.

• Fields inside a table cannot be set for Key field or Field mappings in the settings of a Lookup field.
  Configuring a "Lookup" field
  Fields that can be specified for "Field mappings"
• Fields inside a table cannot be set for Fetch criteria in the settings of a Related records field.
  Field that can be specified for the "Fetch criteria" setting in related records
• When you include references to a field inside a table in the formula of a field that is not in a table, functions that can be used in the formula are only SUM and CONTAINS.
  SUM function: Calculate the total of numeric values
  CONTAINS function: Determine if a field contains an exact match for a specified character string
  List of operators and functions
• If you set Record permalink as a source field in the Field mappings section in the settings of an app action, fields inside a table cannot be set as a corresponding destination field.
  Available field combinations for app actions
• Fields inside a table cannot be set for a Title field.
  Setting Title field
• It is not possible to set permissions for fields inside a table in the Permissions for fields settings.
  Configuring permissions for fields

• If a table contains an Attachment field, data in the table cannot be bulk updated.
  Checking the destination app for file import
• Data in a table cannot be bulk updated if the table contains a Lookup field and you do not have view permission for its Datasource app.
  Checking the destination app for file import

• Tables cannot be placed in a Field group field.
  Field group
• Once you click Activate app or Update app after placing fields, these fields cannot be placed in a table.
• It is not possible to sort data of fields inside a table.
  Which fields and items can be used for "Sort by"?