Marschall is a microbial rennet substitute. As such, it has a wider swath of casein cleavage sites, is more generally proteolytic, than calf rennet. Though from what I've seen, among the microbial substitutes, the protein loss varies, when compared to calf rennet this non-specificity would, ceteris paribus, mean more protein is lost to the whey.
Additionally, from what I've read, in theory, this can lead to a bitterness issue down the road, since many of the peptides resulting from this comparatively more non-specific hydrolysis are known to contribute bitter notes. The jury seems to be out whether in practice this obtains - I know many have said they've no issue with bitterness using these, when making aged cheeses. Perhaps it's a sensory threshold issue; I don't have any data on that.
From what I see, Marschall comes already salted, so I wouldn't know why you need to add more. I have read that microbial rennet is particularly prone to Listeria contamination. I suspect the salt added is used for the same purposes as in calf rennet, preservation.
