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CAM Assist ensures that End mills are never used beyond their usable length. Tool catalogs usually refer to usable length as “LU”, which is often different to both the flute length (APMX) and the overall length (OAL), even for straight shank tools.
In Fusion 360, the usable length (LU) for end mills should be entered into the ‘Shoulder length’ field of the ‘Cutter’ tab (see screenshot below).
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Since this field has almost no effect on the behavior of Fusion 360, you may find that, in your toolsettool library, it has simply been set equal to ‘Flute length'. Since CAM Assist will not use tools that go deeper than LU, this will limit the use of the endmill to a depth equal to the flute length and give you poor results in areas such as deep pockets.
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Assuming that you’ve ‘allowed’ CAM Assist to use the associated tool (using the Tool Use tab) then, if that tool does not have a tool holder assigned in Fusion 360, CAM Assist will assign one so that cutting data parameters can be generated.
However, if you prefer greater control over collision checking and tool choice, you could configure the tool holders yourself in Fusion 360.
Next step
After learning about naming your End mill cutting data and checking the tool definition, the next step is use that information to decide which End mills should be in the toolsettool library?