In the vast majority of his sonnets, Sir Philip Sidney adheres to the so-called "Petrarchan" sonnet form, named after its Italian originator Petrarch. This type of sonnet originated the fourteen-line iambic structure and has the rhyme scheme ABBAABBA CDECDE. In "Sonnet 31" from his "Astrophil and Stella" sequence, the most notable differing feature between it and the Petrarchan form of the others is that it concludes with a rhyming couplet, which is not a Petrarchan feature. You may remember that concluding with a rhyming couplet is a feature more commonly associated with the Shakespearean or "English" sonnet. However, Shakespeare's sonnets have a rhyme scheme of ABAB CDCD EFEF GG, with effectively four quatrains and a rhyming couplet, which usually brings what has come before to a conclusion.
In "Sonnet 31" by Sir Philip Sidney, the poet brings his own twist to the sonnet form, using the entirely different rhyme scheme ABBAABBACDCDEE.