MDL is slo blow to my knowledge.

You want the difference between MDL (slo blow) and AGC (fast blow). Slow blow simply means it takes more time at a given current level for the fuse to break.

In pinball, the current to coils is very 'spikey' and this can blow fast blow fuses, even though the amount of power being driven isn't enough to damage components, so a lot of fuses in a pinball machine are slo blow for this reason.

As a general rule of thumb, a fast blow fuse can be safely used anywhere a slow blow can however it can blow in service too easily.

A slow blow should NEVER be used where a fast blow is specified.