Fillers go in corners where cabinets meet to allow space for drawer handles. From the sound of your question, it seem like you may be in over your head before you've started.

Fillers are used for a number of reasons, not just clearance. Nobody knows your plan. Where is the plan for your kitchen? Not a fancy elevated thing,,,,, the CABINET plan with all the dimensions, that includes the appliances, widows, entry points etc. Post it

They’re all used where needed. Where they’re needed takes knowing cabinet design issues and the specs of the appliances and other features. You need to be working with a Kitchen Designer.

Sometimes people ask questions just because they are curious.

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Types of Moldings for Cabinets

Depending on your design preferences and selected cabinets the options are almost limitless. Here are some of the most common molding options available for your kitchen cabinets:

https://www.cabinets.com/types-of-moldings-for-cabinets

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Best wishes on your new cabinets!

Thank you this is very helpful..