Because pools around the world use different units of measure (meters or yards), when you see a swim workout with numbers and no units -- you just do the workout based on the pool you're in.  If your pool is measured in meters, then it's 400 meters.  If it's measured in yards, then it's 400 yards.  It's that simple!

100, 200, and even 25 would be the same, depending on where you live.  Most of the world measures pools in meters, except the US, which measures in yards.  

Here's are two examples:

Warm Up: 400 at extremely easy effort going 25 your choice drill immediately into 25 swim. Use fins and snorkel throughout.
Translation:  400 meters/yards at extremely easy effort, starting with 25 meters/yards of your choice drill, going immediately into 25 meters/yards swim.


Main Set #1: 3×200 at 85% effort with 20sec rest. 
Translation:  3 x 200 meters/yards at 85% effort with 20 sec rest. between each 200 meters/yards.