"Pricing Type (Optional per Tier)" allows you to use multiple Pricing Types within the same Price Book. The selected "Pricing Type (Optional per Tier)" will be applied only to the tier it is set on; if this value is not filled in, the "Pricing Type (Default)" from the Price Book will apply to the Tier. You can have a mix of Tiers that include the "Pricing Type (Optional per Tier)" and those that don't within the same Price Book.
Minimum
The Minimum allows you to optionally specify a minimum dollar amount for a specific Tier. This is most often used with Linear Pricing Types.
Maximum
The Minimum allows you to optionally specify a maximum dollar amount for a specific Tier. This is most often used with Linear Pricing Types.
Additional Fixed Fee
A value in this field will be added as a fixed amount when this Tier is attained. You have the option at the Price Book level to determine whether these Additional Fixed Fees will be charged at all levels up to the attained level, or just charge the value at the attained level. Price Book level fields will also let you determine whether this value is added to the single rated Charge, or whether a separate Charge is created for this fee.