The Letter of Commitment is marked as optional in the GNeuS application package because it depends on the Research Project submitted by the candidate. Optional does not mean that the applicant can arbitrarily decide whether to submit the LoC for its secondments, but she/he has to meet the following conditions.
When the Research Project proposed by a candidate includes secondments only at institutions that are already GNeuS partner no Letter of Commitment (LoC) is required for the submission.
However, when the Research Project proposed by a candidate includes secondments at institutions that are not already GNeuS partner, for each institution that is not a GNeuS partner the candidate must submit a Letter of Commitment duly filled by an institution representative and signed by its legal representative.
The institutions that are already GNeuS partners, and therefore have already signed the Letter of Commitment, are listed at https://gneus.eu/hosting-institutions.