סנהדרין מז:
"הזמנה לאו מילתא היא"
The Gemara says if one weaves a בגד for a מת (תכריכים), but it has not been used yet for its purpose, אביי says the בגד is אסור בהנאה, because הזמנה מילתא and mere designation is enough to be considered as if the מת was already wrapped in this garment and consequently is אסור בהנאה as all תכריכי המת are. רבא holds the בגד is מותר because הזמנה לאו מילתא and designation alone, is not enoughto consider it תכריכי המת and the garment is not אסור בהנאה until you actually wrapped the מת in it. ר' חסדא says, both are necessary, הזמנה and a מעשה. One without the other is לאו מילתא. Therefore, a bag that was made to hold תפילין (הזמנה), only if after the הזמנה was it actually used for תפילין, would this bag be considered תשמישי קדושה and אסור to keep foreign items in it, like loose change?[דף מח ד"ה ונתנו] תוס' learns that even according to אביי that הזמנה מילתא, it would not work with designating it with דיבור alone, but one would have to be holding the item during the הזמנה בדיבור. The [או"ח ס' מ"ב, ג'] שו"ע paskens like ר' חסדא and says if one was "אזמניה" a bag for תפילין and then put his תפילין in the bag, only then is it prohibited to place money in it as well. If there was הזמנה but it hasn’t been used yet, or it was used for the תפילין but no הזמנה, it is permissible to place money in it. The משנ"ב explains, since we pasken הזמנה לאו מילתא, even if the תפילין bag was made for the תפילין, it is not considered תשמישי קדושה until actually used for the תפילין also.The משנ"ב adds that the "אזמניה" of the מחבר could be with דיבור alone and certainly if one took hold of the bag and then designated it (תוס'), it has the status of תשמישי קדושה. The רמ"א says if one stipulated from the outset that he plans on keeping change or foreign items in his תפילין bag, he may do so.The משנ"ב points out that a טלית bag is not considered תשמישי קדושה as a טלית is not a דבר שבקדושה. The טלית bag is considered תשמישי מצוה and consequently one may keep other items in this bag as well. Even if the larger טלית bag is used to store the תפילין bag in it, the טלית bag would be classified as a תשמיש דתשמיש andמותר for all usage. [או"ח ס' מ"ב,ד'] פסקי תשובות brings a סברא that in our times that we put away our תפילין in covers (boxes) and then place it in the bag, the bag becomes a תשמיש דתשמיש and it is permissible to keep other items in it. Others say, that since theרצועות and knots are outside of the boxes, the outer bag should be considered תשמישי קדושה andאסור to store other items. The משנ"ב says one may not keep his siddur in a bag that is מיוחד for his Tefillin. But "אין למחות" since people do this, it is as if it was stipulated לכתחילה for this. In [ד"ה אסור] באור הלכה he brings in the name of the שער אפרים that any extra space in the bag that is not being used for storing the תפילין, may be used for חול, which would permit using the extra space in the bag, even according to those that point out that the רצועות and knots are outside of its boxes.The משנ"ב points out that a תפילין bag may not be discarded, but is צריך גניזה.