The adjustment only level is the data conversion level that ensures only manual journals will be entered in the secondary ledger. This level means that no data is copied from the primary ledger to the secondary ledger automatically. Instead, the user can enter manual adjustments in the secondary ledger to reflect the differences between the two accounting standards. This level is suitable for scenarios where the accounting methods or charts of accounts are very close between the primary and secondary ledgers, and there is no need to maintain detailed journals or subledger transactions in the secondary ledger. References: Primary Ledgers, Secondary Ledgers, and Reporting Currencies, High Volume Data Migration Considerations for General Ledger, Reporting Currencies - Conversion Level Balance

To achieve the requirement of only single currency journals being entered for this company, you should enable "Limit a Journal to a Single Currency" on both ledger options and journal sources. Ledger options are settings that apply to a specific ledger, such as journal processing options, currency options, and average balance processing options. Journal sources are identifiers that indicate where a journal originated, such as manual entry, subledger accounting, or import. You can enable "Limit a Journal to a Single Currency" on both ledger options and journal sources using the Specify Ledger Options and Manage Journal Sources tasks in Setup and Maintenance. This will enforce single currency journals for journals entered on the Create Journal page and for journals that are imported. You do not need to enable "Limit a Journal to a Single Currency" on journal lookup codes, as these are codes that indicate the status of a journal, such as Entered, Posted, or Reversed. You do not need to enable "Limit a Journal to a Single Currency" on journal category, as this is an attribute that classifies journals by purpose or function, such as Purchase Invoices or Allocations.
You do not need to enable "Limit a Journal to a Single Currency" on profile option, as this is a setting that affects the behavior of an application or feature for a user or responsibility. Reference: Oracle Financials Cloud: General Ledger 2022 Implementation Professional Objectives - Define Ledgers 12

To add a global branding logo and more predefined transactional attributes to the journal approval email notification, you should copy (or customize) and edit the layout template and the data model. The layout template is a file that defines the appearance and content of the notification, such as text, images, tables, or charts. The data model is a file that defines the data sources and queries that provide data for the notification, such as predefined transactional attributes. You can copy (or customize) and edit the layout template and the data model using Oracle Analytics Publisher reports. You do not need to copy (or customize) and edit the output type, as this is a setting that determines the format of the notification output, such as HTML or PDF.
You do not need to copy (or customize) and edit the style template, as this is a file that defines the styles and formatting of the notification elements, such as fonts, colors, or margins. You do not need to copy (or customize) and edit the sub template, as this is a file that contains reusable content or logic that can be referenced by multiple layout templates. Reference: Oracle Financials Cloud: General Ledger 2022 Implementation Professional Objectives - Configure Workflow Approvals and Notifications 12