The units you specify are not overwritten when you open a scene file with different units. The first time you run Maya, the Default Working Units are set from the Working Unit values in your preferences.
Sets the working time unit for animation. The Time Slider displays time values in the unit chosen. The Time Slider values update when you change the time units. You can specify the time unit as frames or as clock values.
Frame selections include: 15 fps, Film (24 fps), PAL (24 fps), NTSC (30 fps), Show (48 fps), PAL Field (50 fps), NTSC Field (60 fps), 2, 3, 4, 5, 6, 8, 10, 12, 16, 20, 40, 75, 80, 100, 120, 125, 150, 200, 240, 250, 300, 375, 400, 500, 600, 750, 1000, 1200, 1500, 2000, 3000, and 6000 frames per second.
The terms frame and time refer to working time units as displayed in the Time Slider. Technically the term frame applies only when Time is specified in frames per second (fps). In general, time can refer to frames or to clock values.
By default when you change the current time unit, the times for any existing keys are modified so that playback timing is preserved. For example, a key set at frame 12film changes to frame 15ntsc when the current time unit is changed to NTSC, since they both represent a key at 0.5 seconds. When this option is on, it leaves the key at 12ntsc that was originally at 12film. The default setting for this option is off. The option turns on once the current time unit is changed.
The Tolerance value determines the degree of accuracy that is maintained between the original and fit (or interpolated) curves. This setting applies globally to Maya. You can change it on a case-by-case basis. Set the following tolerance options: