Kli SheMelachto LeIsur

In the laws of Shabbat, Kli SheMelachto LeIsur (A tool whose defined use involves work forbidden) is one of the types of muktzeh, which is - an item that is typically used for activities that are forbidden on Shabbat. Under certain conditions, the sages permitted the use of these utensils, provided that they are intended for a use that is allowed on Shabbat.

Background
The regulation that restricted the carrying of "A tool whose defined use involves work forbidden" is attributed to Nehemiah, as it appears in the verse. The prohibition on utensils whose primary use is prohibited was not to carry them at all, even for their own use or for the use of their place, because Nehemiah saw that people were lenient with the laws of Shabbat and were carrying from a private domain to a public domain, thereby violating the prohibition of carrying, which is one of the thirty-nine primary categories of labor. Therefore, to create a safeguard, they completely prohibited the carrying of all utensils in the house. After Nehemiah saw that the observance of Shabbat had strengthened among the people, they again permitted the carrying of utensils whose primary use is permitted, and after a period, they even permitted the carrying of utensils whose primary use is prohibited for their own use or for the use of their place.

Permits for Using a Tool Whose Primary Function is Prohibited
A household hammer is a tool whose primary function is prohibited, but it is permitted to move it for its own use - (for example, cracking nuts) or for its place.


 * For its own use - to use it in a manner that is permitted on Shabbat by the object's body. For example: although a hammer is a tool whose primary function is prohibited because it is used for building, it is permitted to use it on Shabbat to crack nuts.
 * For its place - to move it from a certain place because that place is needed. For example: if a tool whose primary function is prohibited is left on a sofa. When one wants to sit on that sofa, it is permissible to move that object.

Additionally, even if one wants to save the tool itself or use it in a manner that is not permitted on Shabbat, one can also find a permissible use for the tool even if that is not the main purpose. For example: one can bring a phone inside from the yard where it is raining, provided that the phone itself is used. Another way to move a tool whose primary function is prohibited to save the tool itself (such as from sun to shade) is by placing a non-muktzeh object on it. When moving a tool whose primary function is prohibited in one of these two ways, it can be placed wherever one wants and there is no obligation to leave it immediately. A tool whose primary function is prohibited is only such a tool that one does not mind using for another purpose, like a hammer. Any tool whose primary function is prohibited and one is particular not to use it for any other purpose besides its intended function is considered muktzeh due to monetary loss. The examples given in the Shulchan Aruch are: a slaughtering knife and a circumcision knife, scribal tools, and the like. For this reason, any expensive device that is not moved from its place for any purpose other than its intended function, such as sensitive electrical devices, is considered muktzeh due to monetary loss.

A utensil whose primary use is for prohibited and permitted activities
A utensil that is used for both permitted and prohibited activities, if its primary use is for prohibited activities and only occasionally used for permitted activities, it is considered a utensil whose primary use is for prohibited activities. However, if it is made for both prohibited and permitted uses, it is allowed to be moved on Shabbat and is treated as a utensil whose primary use is for permitted activities, which means it can be moved for any purpose, even to protect the utensil itself (but not without any purpose at all).