What those posting since my question have said is what I had assumed was meant by the sentence I quoted namely: it means that it can be exposed at EI 50 without any or only minimal loss of shadow detail. Presumably there must be some loss, otherwise it may as well be classified as ISO 50 and not 25 unless Rollei feels that there is a marketing advantage to label it as 25. However the use of the word development without adding the word "times" leaves uncertainty and quite what "development" means cannot be deduced without some form of specification sheet.

So do you simply change developer, development regime as in agitation frequency, extend development or do you use the same development times as you do with T-Max 400 when it is rated at 800?