Hi,
As per my understanding, When we are issuing stock to reservation, you need to determined batch with reference to SLED and vendor specific lot. please correct it if anything wrong.
Based on SLED, you can go with FIFO method .. to determined the which ever you are received first that will be the issuing first.
Vendor specific lot, you have to go with user exist while issuing.
MBCFC004 Maintenance of batch specifications for goods movements