ACES does not mandate a particular workflow, but there is a fairly common ACES-based workflow that is flexible enough to meet most production needs. In this workflow, there are four points at which the color values in the captured scene can be changed for creative or for technical reasons: in a show look; in an on-set grade; in a VFX pre-grade; and in the final DI grade. Again, this is not a formal recommendation from the ACES development team, and even when adopted, each step is optional (though only live broadcast is likely to omit the final DI grade). If the workflow sounds familiar, it is because this type of workflow was established for high-end VFX-heavy productions long before ACES was developed.
Looks are color changes that are applied uniformly across the captured frame, and commonly are applied to many if not all shots in a production. An example look might be one emulating a particular film stock as developed and printed by a particular lab. Looks are typically developed during pre-production by the cinematographer and DIT, with input from other creative staff -- for example, the final DI colorist is often involved in look development.
Technically speaking, looks are applied to data in the linear ACES color space. For looks analytically derived from physical measurements (as would be the case for the print film emulation mentioned above) this causes no difficulty, but a linear space is not particularly colorist-friendly. So looks developed interactively by colorists are often developed in a colorist-friendly color space like ACEScc, and wrapped transparently in ACES-to-ACEScc and ACEScc-to-ACES transformations.
On-set grading is, typically, the application of ASC CDL operations — slope, offset, power and saturation changes — uniformly across the frame to log-encoded ACESproxy data either on-set during the composition of a shot, or shortly after the shot is captured in near-set dailies production. (In this sense, on-set grading is something of a misnomer; it happens as much near-set as on-set.) On-set grades are applied before any look is applied.
Pre-grades are for shot-specific overall changes, typically to deal with uncompensated changes in illuminant over long periods of time. More detail on pre-grading is available in the post-production section, though it is worth noting here that the artists using the pre-graded plates should be provided with the on-set grades as well.
The final DI grade is the last opportunity for creative change to the look of the image prior to distribution. More detail on the final DI grade is available in the post-production section, though it is worth noting here that the final DI colorist should be provided with the on-set grade, where it may be used as a starting-point grade, or at least as a historical reference.