Hi ​@Cboy , The sales part should be with sales type ‘Sales and Rental’ in order to add a complementary part. As per current design, complementary parts cannot be added to rental only parts and it is a known limitation.  If a particular user added a quantity for a complementary part whilst being on the rental parts tab, the complementary part would be added to the Order Lines tab and not the rental part tab. What you have mentioned is an enhancement request and you can find the detailed KBA in KB0062548.
 

Hope this helps. 

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Thanks,

Sayuri.

The IFS help documentation shows Rental Line complimentary parts will be added to the rental lines tab. Is this an upgrade issue only in Cloud? Did this work previously? The documentation is misleading as it reads like functionality to add complimentary parts to rental tab should already be available.