Size 13 seed beads are very small beads. Made from glass in the Czech Republic, these size seed beads are commonly used in bead weaving to give patterns and designs more detail. These can be used for simple and more complicated bead stringing as well. These beads are very small and require a size 12 or 15 needle.
These Czech glass size 13 seed beads are more similar to a Japanese size 15, so if you pattern requires a size 15 Japanese seed beads you should be able to substitute these without it affecting your project to much. It is wroth baring in mind every pattern is different so a straight swap will not necessarily work fir everything, definitely worth an experiment.