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Having two people or programs opening and modifying the same file/document/database could obviously cause problems. One program reads a record, the other reads the same record, the first one makes a change and saves it, the second makes a different change and saves the record again, overwriting the first change. With databases having pointers between various records, inconsistant updates could destroy the integrity of the data. Fancy database systems have ways around this problem. But for what is essentially a single user product, there is no real reason for two progams to be updating the same database at once. So the system just stops the second attempt from reading the database. It is a fairly crude way of preventing you from damaging the database.
When installing any version of UFT, the install program lets you specify which directory the installation will use. It gives a suggestion, but your could change that before the actual install is done. You could even have two copies of the same version if you wanted. In general, when installing a new version, you should not overwrite the old one, just because of the bugs that may be in the new one. Most modern computers have lots of disk space and can keep several versions around. You could also have Roots 4 and/or any of its successors installed as well, all accessing the same project(s). But just one at a time.