groff has a "safer mode," which disables its .open, .opena, .pso, .sy, and .pi requests, and an "unsafe mode," which enables them. The .U register is 0 in safer mode and 1 in unsafe mode.
In Heirloom troff, those requests are always enabled, so it is essentially always in the equivalent of groff's unsafe mode. Therefore, for compatibility with groff code that checks the .U register, the g tmac file should set it to 1.
groff has a "safer mode," which disables its
.open,.opena,.pso,.sy, and.pirequests, and an "unsafe mode," which enables them. The.Uregister is 0 in safer mode and 1 in unsafe mode.In Heirloom troff, those requests are always enabled, so it is essentially always in the equivalent of groff's unsafe mode. Therefore, for compatibility with groff code that checks the
.Uregister, thegtmac file should set it to 1.