So the planned arm is “ABC” if I understand it well.
If the subject retreated after randomization but before actual treatment, the value for ACTARMCD is “NOTTRT” (“not treated”).
If however the subject retreated after first treatment, ACTARMCD is still “ABC” and there will be a record in DS with the reason for / explanation of the discontinuation.
See SDTM-IG 3.2 page 64.