Unrecaptured Section 1250 is the gain due to due depreciation, which is taxed at regular tax rates up to 25%.

For an Installment Sale, the taxable amount for the principal FIRST takes the Unrecaptured Section 1250 Gain, before any long-term capital gains.

For example:

Let's say some real estate was bought at $100,000, sold at $120,000 and took $5,000 of depreciation.  Out of the total gain of $25,000, the $5000 is Unrecaptured Section 1250 Gain.

First year of the Installment Sale received $2000 of taxable principal payments.  That $2000 is Unrecaptured Section 1250 Gain, taxed at regular rates up to 25%.  The other $3000 is carried to next year.

Second year of Installment Sale received $4000 of taxable principal payments.  The first $3000 is Unrecaptured Section 1250, the last $1000 is long-term capital gain.

Subsequent years of Installment Sale.  All taxable portion of principal payments will be long term capital gain because the Unrecaptured Section 1250 Gain already been used/taxed.

Does that help?