In the fitt principle, the choice of swimming instead of walking would be an example of in the fitt principle, the choice of swimming instead of walking would be an example of time intensity frequency type

swimming instead of walking would be an example of TYPE in the FITT principle.

This can be easily determined if you keep the below explanation in mind about the FITT cycle:

Frequency: This is how often we train. How often our sessions are planned (e.g. in a month, week etc.)

Intensity: this is how hard you work out.  

Time: How long your training sessions are or length of the activities that take place during the training session.

Type: This is the training method e.g. weight training, flexibility, sprint etc.  

So if a person is swimming instead of walking he/she is changing the exercise/ training method, whilst all other factors in the FITT cycle remained the same.