If you want to apply conditional validation on 'Maximum Marks' or 'Obtained Marks', then you need to add 'Type' input as '8CS5A'
{{subjectdetailsForm.controls?.subjectCode?.errors?.maxNumber.message}}
To apply the validation of Max Number on 'Obtained Marks' field, then enter the value '8CS5A' in 'Subject Code' textbox and you can enter maxNumber upto 100
Conditional Expression with type 'string'
{{subjectdetailsForm.controls?.obtainedMarks?.errors?.maxNumber.message}}