Am NOT Sure - I think you could see the simplistic filling of a LHS position in training management as follows: ideally internally upgrade before using external sources as you are putting a "known quantity" into the left hand seat, with a training followed and assessed over yeas in the company vs an unknown sourced entity.
To achieve this a new FO needs to be recruited and trained to enable (after line training) a SFO to proceed onto the upgrade process. Once passed he can take the available LHS position. Should a candidate fail this process, then another SFO proceeds until one is successful.
Should the requirements for LHS pilots exceed training capability/capacity as described above, due LHS outflow, simulator availability or fleet expansion, time has to be saved somewhere in the process.
This time saving can be achieved by using DECs, as they will occupy the available seat the new FO would take in the classroom for type conversion, but then (after line training) these DECs are immediately the LHS the company requires, without the need for the SFO process after a new FO.
Thus DECs are reducing the training time requirement/capacity by the time/capability required for converting SFOs.
Advantage: cost, time and capacity saving in training management.
Disadvantage: upsetting SFOs waiting for their "turn" for promotion.
Sometimes companies, even after meticulous training management planning, are faced with a projected future shortfall in LHS using their preferred method of internal upgrades.
Options available are to reduce SFO upgrade passing requirements/standards, thus increasing SFO passing rates to LHS and reducing slightly the capacity required to fill LHS, or resort to DECs for some time. If additional simulator capacity exists, using that can raise output, but in absence the DEC route is sometimes only option.