Currently, HCMTLMU01 ( Employee times ) is a temporal join data source, which uses characteristics 'key date' as the key member of its Query and cannot be used for join data sources as it would effect the overall result once the Query is fired.
Considering the scenario where this data source was enabled to be used in joins, we would have inconsistent results due to the dependency of HCMTLMU01 with its query element and the dependency on temporal element. As most of the joins considering HCM scenarios with Time x. Any other units is not measured by dates, we would certainly have misleading results once such customized data source would be used.
Therefore, it was decided that Datasource HCMTLUM01 cannot be used as Join Member as it uses a temporal join. however it can still be used for combined data sources.
In case you are looking for specific customization, you can contact service center ...firstname.lastname@example.org
Additional information, The data sources that currently contain temporal joins in the system