Either one is possible, provided the dp in the transfer line from the kettle reboiler overflow sump to the product sump in the column is low. The dp should be such that the level difference between kettle and column is say less than 3-5%.

https://www.gas-compressor.info/reboiler/kettle-re...

Hi,
You may consider the link above .
Note : You should have posted this query in another activity like process engineering .
My view,
Pierre

My earlier response assumes your query refers to the level between the product sump in the reboiler to the product sump in the column.

Another arrangement may be where the overflow weir in the kettle is flooded ie all the liquid in the reboiler is at a common level. In this case, it would be preferable to use the level in the kettle for level control, since there may be high dp across the tube bundle in the kettle. This will prevent exposing any tubes in the kettle, which would otherwise result in mechanical failure of the exposed tubes due to differential thermal expansion.

Can you post a diagram of the arrangement in the column bottom and reboiler kettle, with levels and overweirs shown also ?