In law hold for another agency means that the inmate or person is to be held by the jail for another county or state as the person has charges against them in those places.
If you're being held for another agency then you won't be able to bail out until your current charges are taken care of with the other county or state jail court system.
Hold for another agency in law means that the person who has been arrested in one place has a warrant in another place.
So, when they are bonded out of wherever they are, they will immediately be transferred to the location where the other warrant is.
For example if the person got arrested in Decatur, but has a warrant out for their arrest in Atlanta.
Whenever he or she bonds out of Decatur, Atlanta PD will be notified to come pick the person up from Decatur for transfer to Atlanta jail.
The only way to avoid this transfer is to have the the warrant problem handled before he or she is bonded out of wherever they are now.
However you can usually bond out of the other jail if you have a bond after being arrested by the other agency.