imageinfo() is just a convenience tool for what imfinfo provides. That said, the metadata that's available depends on the file format and the software that wrote the file, so even the documentation for imfinfo() can't account for every possible tag that might show up. Offset information is common in metadata. Sometimes it's spatial offsets in the image space. Sometimes it's data offsets to the beginning of a certain field or data block.
In this case, I'm going to guess that this is a TIFF file and the tag specifies the byte offset to the end of this image block. I'm not really sure whether that's from the beginning of the file or if it's relative to the IFD position. I'm not terribly familiar with the format specification for TIFF. A more precise (and certain) description would require more information about the file and perhaps some time reading the format spec.
