As introduced in previous units, downtime-optimized conversion requires a customer buffer that contains the relevant finance (FI) customizing to be applied by Software Update Manager. This customizing has to be manually created after a standard conversion run and is saved in a transport. This transport is then included in the downtime-optimized conversion run using Customer Buffer Integration.
This means, the content of the Customizing of the transport is applied to the system on which the downtime-optimized conversion is executed and therefore needs to fit to the actual Customizing present in the system. Typically, the initial Customizing transport is created rather early in the project, for example, after a sandbox run. Until production system is converted, time passes in which - very likely - relevant Customizing on production system changes.
To avoid having to run another standard conversion on a copy of production to create a Customizing that fits to production again, you may adapt the relevant FIN customizing directly in the temporary (TMP) instance. Software Update Manager stops in a dialog in phase REQ_DOCMAX_DELTA_CUSTOMIZING and Customizing can be adapted. Please note that this approach does not enable adaption of Customizing in general, but only a selected subset supported by the reference Implementation Guide (IMG). The updated Customizing is saved into a transport, which can be used for future downtime-optimized conversion runs. The Customizing transport is not released during the run, but after the conversion.
For more information on Uptime Customizing for FI Conversion, see SAP Note 3444013, the guide for downtime-optimized conversion and following community blog: https://community.sap.com/t5/technology-blog-posts-by-sap/simplify-the-downtime-optimized-system-conversion-to-sap-s-4hana-with/ba-p/14101686.