Use of start() of sequences in UVM

The prototype of start() is:

virtual task start ( uvm_sequencer_base sequencer,
                     uvm_sequene_base   parent_sequence = null,
                     int                this_priority = -1,
                     bit                call_pre_post = 1       );

The first argument must be set, others being optional. After calling seq.start(), following methods will execute in order and in demand:

seq.pre_start();
seq.pre_body();                 // if call_pre_post == 1
    parent_seq.pre_do();        // if parent_seq != null
    parent_seq_mid_do(this);    // if parent_seq != null
seq.bosy();
    parent_seq_post_do(this);   // if parent_seq != null
seq.post_body();                // if call_pre_post == 1
sub_seq.post_start();

Reference:

[1] https://www.chipverify.com/uvm/how-to-execute-sequences-via-start-method