Do you mean current balancing, as in spreading a load across several transistors?
Components can have slightly different specs. One transistor can hog current and burn up.
Balancing needs to be done so that all transistors get the same share of the load.
This is usually done by putting a low-ohm resistor inline with each transistor.
As an alternative you can fine tune each bias until all transistors are balanced. However it's error-prone and can be thrown off by later changes in load, or supply, or drift in operating specs.
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There is also current matching as in testing transistors/mosfets in order to find matching performance among a pair, trio, etc.
And it doesn't have to be for parallel load balancing. It may be to match a PNP transistor to an NPN type, in order to get balanced performance in an AB class amplifier.