The Atacama Compact Array (ACA) will be automatically selected by the Observing Tool (OT) when needed (see the section "Imaging Considerations - Arrays and Configurations" in the ALMA OT User Manual). The OT will decide whether the ACA is needed, based on the PI-requested "largest angular scale" (LAS). This value is compared to the "Maximum Recoverable Scale" (MRS) of the 12-m Array (as listed in Table A-1 of the ALMA Proposer's Guide). If LAS>MRS, then the ACA will be used. This selection cannot be overridden by the user.
There is also the possibility to request ACA stand-alone observations using the appropriate button. TP stand-alone observations are not permitted.
Note that Total Power (TP) observations are only allowed for spectral line observations in Band 1, 3-8. For Band 9 and Band 10 observations or continuum observations in any band, the TP array is not allowed and such projects requesting the ACA must additionally have a PI-requested LAS that is less than the Maximum Recoverable Scale for the ACA 7-m Array (as listed in Table A-2 of the ALMA Proposer's Guide).
For more details, see the relevant section in the ALMA Technical Handbook.