First, I just want to clarify that MAXTRANSFERSIZE is not a NetBackup-specific code/parameter.
Rather, it's a standard parameter used by Microsoft SQL virtual device (VDI) during backup/restore. Pretty much any backup tool that backs up Microsoft SQL database can use this parameter for tuning purpose. In older NetBackup version, the default value we used was 64KB (the minimum) unless user specified otherwise. Newer NetBackup version (6.5.x, 7.x.x) defaults to 4MB (the maximum) as it improves backup performance and more servers have bigger physical memory.
The following links should hopefully help you understand the parameter better: