The best description I could find:
Quote
THE porcelain called reticulees comes into the category of blue and white because some of it was decorated with blue under the glaze. The pieces have double walls, of which the outside one is pierced with a pattern, often a network of exceeding fineness, a lace-work in porcelain through which may be seen the exquisite blue design upon the internal wall. It seems just as if the potter had two pieces made to fit at the top and bottom only. On the foundation piece—the internal wall—he expended the art of decoration in blue. On the other piece, even more care must have been shown in cutting the clay into delicate tracery, so minute as to be marvellous. Then the fitting of the two walls together was completed, and finally the firing process took place. Such pieces are pre-eminent in curiosity, in interest and in skill, and their variations are wonderful. Some are only blue and white, some are blue and white with reserves in biscuit. All had the outside wall pierced with a pattern.