The Lord Mayor Locum Tenens (LMLT) is a Senior Alderman past the chair who stands in for the Lord Mayor when the incumbent finds themselves on duty outside of London and where an invitation has already been accepted. The two parties cannot be present in the City at the same time.
If an LMLT must be appointed this is decided by Mansion House, not the hosting organisation or Livery although if a senior Alderman has a connection with the Livery concerned it is likely they will be appointed.
An alternative is a Representative Lord mayor who may be appointed by Mansion House and can be in the Coty at the same time as the Lord Mayor. They may also be someone other than a past Lord Mayor such as a Sherriff.
The reason for these clear boundaries are largely due to support and costs. The LMLT will have the same rights and privileges as the Lord Mayor. These may include a speech writer, chauffeur, duty esquire etc.
It may be useful to note that the correct pronunciation of Locum (low) Tenens (10)